Wireless communication system, wireless communication device and wireless communication method, and computer program

ABSTRACT

Access control based on CSMA is favorably carried out with the RTS/CTS method used together. 
     If the RTS/CTS procedure is used together, CTS information is transmitted in response to the reception of RTS information, and data is transmitted in response to the reception of CTS. The CTS transmitting station measures the quality of RTS receive signal, and thereby determines a transmission rate at which it can receive and notifies a station as the destination of CTS. The station as the destination of CTS transmits data in response to CTS information. As the transmission rate for this data, the transmission rate indicated in the RATE field in CTS is applied.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. application Ser. No. 10/916,596, filedAug. 12, 2004 and claims the benefit of priority under 35 U.S.C. § 119of Japanese Patent Application No. 2003-364231, filed Oct. 24, 2003. Theentire contents of these applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system whereina plurality of wireless stations communicate with one another as inwireless LAN (Local Area Network) or PAN (Personal Area Network), awireless communication device and a wireless communication method, and acomputer program. More particularly, it relates to a wirelesscommunication system, a wireless communication device and a wirelesscommunication method, and a computer program wherein random access ismade based on carrier detection by CSMA/CA (Carrier Sense MultipleAccess with Collision Avoidance).

Further more particularly, the present invention relates to a wirelesscommunication system, a wireless communication device and a wirelesscommunication method, and a computer program wherein access control iscarried out based on CSMA using the RTS/CTS method to maintaincommunication quality. Further more particularly, it relates to awireless communication system, a wireless communication device and awireless communication method, and a computer program wherein aplurality of classes of frames, such as RTS, CTS, DATA, and ACK, aremultiplexed and thereby flexible transmitting and receiving proceduresare provided and overheads are reduced.

2. Description of the Related Art

The wireless network has received widespread attention as acommunication system which relieves users of cable wiring between wirecommunication devices. With the wireless network, the majority of cablescan be omitted in working spaces, such as offices; therefore,communication terminals, such as personal computers (PCs), can berelatively easily relocated. As wireless LAN systems become faster andinexpensive, recently the demand for the wireless network has beensignificantly increased. In particular, these days, the introduction ofpersonal area network (PAN) has been considered to build a small-scalewireless network among a plurality of pieces of electronic equipmentexisting in people's surroundings to carry out informationcommunication. For example, different wireless communication systems andwireless communication devices are defined using frequency bands, suchas 2.4-GHz band and 5-GHz band, for which licenses from competentauthorities are unnecessary.

Typical standards related to wireless network include IEEE (TheInstitute of Electrical and Electronics Engineers) 802.11 (Refer toNon-Patent Document 1, for example.), HiperLAN/2 (Refer to Non-PatentDocument 2 or Non-Patent Document 3, for example.), IEEE802.15.3, andBluetooth communication. With respect to the IEEE802.11 standard,various wireless transmission methods, such as the IEEE802.11a standardand the IEEE802.11b standard, are present depending on differences inwireless communication method and frequency band used.

To build a local area network using radio engineering, the followingmethod is generally used: one device, designated as “access point” or“coordinator,” that functions as control station is provided in thearea. Then the network is formed under the centralized control of thecontrol station.

In wireless networks provided with an access point, an access controlmethod based on bandwidth reservation is in wide use. In this method,when information is transmitted from some communication device, a bandrequired for the information transmission is reserved at the accesspoint. Thus, transmission channels are utilized so that collision withinformation transmission by other communication devices will beprevented. That is, by providing an access point, synchronous wirelesscommunication, wherein the communication devices within the wirelessnetwork synchronize them with one another, is carried out.

However, a problem arises if asynchronous communication is carried outbetween a transmitting communication device and a receivingcommunication devices in a wireless communication system provided withan access point: wireless communication must be carried out through theaccess point without exception, and thus the efficiency of transmissionchannel utilization is reduced by half.

To cope with this, “ad-hoc communication” in which terminalsasynchronously communicate directly with each other has been devised asanother method for building a wireless network. In particular, insmall-scale wireless networks consisting of a relatively small number ofclients located nearby, the ad-hoc communication is believed to beappropriate. In the ad-hoc communication, any terminals can carry outwireless communication directly, or randomly, with each other withoutuse of any specific access point.

In ad-hoc wireless communication systems, a central control station doesnot exist; therefore, it is suitable for constituting, for example, ahome network comprising household electric appliances. The ad-hocnetwork has the following features, for example: even if one unitbecomes faulty or is turned off, routing is automatically changed, andthus the network is less prone to fail; packets are caused to hop aplurality of times between mobile stations, and thereby data can betransmitted a relatively long way with a high data rate maintained.Various cases of ad-hoc system development have been known. (Refer toNon-Patent Document 5, for example.)

For example, the IEEE802.11 wireless LAN systems are provided withad-hoc mode in which the system autonomously and decentralizedlyoperates in a peer to peer manner without providing a control station.

With respect to wireless LAN networks in the ad-hoc environment, it isgenerally known that a problem of hidden terminal occurs. Hiddenterminal is a communication station which can be heard by onecommunication station as the counterpart of communication but cannot beheard by the other when communication is carried out between thespecific communication stations. Hidden terminals cannot carry outnegotiation between them, and thus transmit operation can collide.

As one of methodologies for solving such a problem of hidden terminal,CSMA/CA according to the RTS/CTS procedure is known.

The CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) isa connecting method wherein multiple access is made based on carrierdetection. In wireless communication, it is difficult for acommunication device to receive signals by which the device itselftransmits information. Therefore, the communication device confirms thatthere is no information transmission from other communication devices bythe CSMA/CA (Collision Avoidance) system, not by the CSMA/CD (CollisionDetection) system. Then, the communication device starts its owninformation transmission, and thereby collision is avoided. The CSMAsystem is an access method suitable for asynchronous data communication,such as file transfer and electronic mail.

In the RTS/CTS method, the communication station as the origin of datatransmission transmits a request to send packet RTS (Request To Send).In response to the reception of a response packet CTS (Clear To Send)from the communication station as the destination of data transmission,the transmitting communication station starts data transmission. Whenhidden terminals receive either or both of RTS and CTS, they set theirown transmission stop period. The transmission stop period is equivalentto a period during which data transmission is expected to be performedbased on the RTS/CTS procedure. Thus, collision can be avoided. Hiddenterminals for the transmitter station receive CTS and set a transmissionstop period to avoid collision with data packets. Hidden terminals forthe receiver station receive RTS and stop a transmission period to avoidcollision with ACK.

Here, the wireless network will be described with IEEE802.11a, anexpansion standard to IEEE802.11, taken as an example.

FIG. 15 illustrates the frame format specified by IEEE802.11a. In thefigure, a preamble which indicates the presence of packet is added tothe head of each packet. For the preamble, known symbol patterns aredefined in the standard. Based on these known patterns, receivers canjudge whether a receive signal corresponds to a preamble or not.

Following the preamble, SIGNAL field is defined. In the SIGNAL field,information required for decoding the information portion of the packetconcerned is placed. The information required for decoding packets isdesignated as PLCP header (Physical Layer Convergence Protocol header).The PLCP header includes: RATE field which indicates the transmissionrate of an information portion (This includes Service field as part ofPLCP header; however, it is hereafter simply and generically referred toas “information portion” for the purpose of simplifying theexplanation.); LENGTH field which indicates the length of informationportion; parity bit; Tail bit of encoder; and the like. Receivers decodethe subsequent information portions based on the result of decoding theRATE and LENGTH fields in the PLCP header.

The SIGNAL portion where the PLCP header is placed is subjected toencoding resistant to noise, and is transmitted at 6 Mbps. In case ofnormal packets, the information portion is transmitted in a transmissionrate mode in which the highest bit rate is provided to the extend thatthe occurrence of errors is minimized according to the SNR or the likeof the receiver.

IEEE802.11a defines eight classes of transmission rate modes: 6, 9, 12,18, 24, 36, and 54 Mbps, and any of them is selected. If atransmitter/receiver is positioned nearby, a high-bit rate transmissionrate mode is selected. A communication station positioned far awaycannot decode this information sometimes.

The information portion is transferred as PSDU (Physical Layer ServiceData Unit) to the data link layer as the higher level layer. FIG. 16illustrates the constitution of the PSDU frame field. IEEE802.11 definesseveral frame types. Here, description will be given only to four typesof frames, RTS, CTS, ACK, and DATA, required for describing the presentinvention.

For each frame, Frame Control field and Duration field are defined incommon. The Frame Control field holds information indicating the typeand use of the frame concerned, and more specifically, it describes theinformation listed in Table 1. The Duration field holds information onthe use of NAV (Network Allocation Vector) (to be described later), anddescribes the time that elapses before the transaction of the packetconcerned is completed.

TABLE 1 Length Field Name [Bit] Description Protocol Version 2 Versioninformation Type/Subtype 6 Identifier indicating the frame type ToDS,FromDS 2 Identifier determining what each ADDR indicates More Fragment 1Flag indicating the last fragment Retry 1 Flag indicating whether thisis retrans- mission or not Power Management 1 Flag indicating powermanagement mode More Data 1 Flag indicating whether more data has beenaccumulated or not WEP 1 Flag indicating the utilization of WEP Other 1

In addition to the foregoing, the Data frame contains: four addressfields Addr1 to Addr4 for identifying the communication stations as theorigin of transmission and the destination and others; sequence field(SEQ); Frame Body which is the primary information to be provided to theupper level layer; and FCS (Frame Check Sequence) which is a checksum.

In addition to the foregoing, the RTS frame contains: Receiver Address(RA) which indicates the destination; Transmitter Address (TA) whichindicates the origin of transmission; and FCS which is a checksum.

In addition to the foregoing, the CTS frame and ACK frame contain RAwhich indicates the destination and FCS which is a checksum.

Here description will be given to the method for coping with accesscontention defined in IEEE802.11.

IEEE802.11 defines four types of inter frame spaces (IFS): SIFS (ShortIFS), PIFS (PCF IFS), DIFS (DCF IFS), and EIFS (extended IFS) inascending order.

IEEE802.11 adopts CSMA as a basic media access procedure (describedabove). Before a transmitter transmits something, the state of themedium is monitored and further a backoff timer is actuated for a randomtime. The transmitter is provided with transmission right only whenthere is no a transmit signal during this period, and the transmittercan transmit packets on the medium.

When a normal packet is transmitted according to the CSMA procedure, theDCF (Distributed Coordination Function) operate as follows: after thetransmission of a packet of some kind is completed, the state of themedium is monitored only for a DIFS. If there is no transmit signalduring this period, random backoff is carried out. If there is notransmit signal during this period, either, then transmission right isprovided.

When a packet, such as ACK, whose degree of urgency is exceptionallyhigh is transmitted, it is permitted to transmit the packet after ashort inter frame space SIFS. Thus, packets whose degree of urgency ishigh can be transmitted before packets transmitted according to thenormal CSMA procedure.

To sum up, the reason why different types of inter frame spaces IFS aredefined is as follows: packets competing for transmission right aregiven priorities according to whether their IFS is SIFS, PIFS, or DIFS,that is, according to the length of inter frame space.

Next, description will be given to the RTS/CTS procedure in IEEE802.11,referring to FIG. 17 to FIG. 19. With respect to wireless LAN networksin the ad-hoc environment, it is generally known that a problem ofhidden terminal occurs. CSMA/CA according to the RTS/CTS procedure isknown as one of methodologies for alleviating most of this problem(described above). IEEE802.11 also adopts this methodology.

FIG. 17 schematically illustrates an example of the operation of theRTS/CTS procedure. This figure shows an example in which information(Data) of some kind is transmitted from STA0 to STA1.

Prior to the actual transmission of information, STA0 transmits an RTS(Request To Send) packet to STA1 as the destination of informationaccording to the CSMA procedure. In response to the reception of the RTSpacket, STA1 transmits a CTS (Clear To Send) packet to STA0, feedingback that STA1 has successfully received the RTS.

If the transmitting STA0 successfully receives the CTS packet, themedium is considered to be clear, and the transmission of information(Data) packets is immediately started. When STA1 has successfullyreceived the information (Data) packets, STA1 returns an ACK packet.Thus, the transmission and reception transaction equivalent to onepacket is completed.

FIG. 18 illustrates action which can take place in surrounding stationswhen the RTS/CTS procedure is carried out between a transmitter stationand a receiver station. In this figure, the following communicationenvironment is assumed: four communication stations, STA2, STA0, STA1,and STA3, exist, and communication stations adjoining to each other inthe figure are located within the range of radio waves. Here, it isassumed that STA0 desires to transmit information to STA1.

The transmitting STA0 confirms that the medium is clear for a certainperiod (from time T0 to time T1) according to the CSMA procedure. Then,STA0 starts the transmission of an RTS packet to STA1 at time T1. In theType/Subtype field of the Frame Control field of the RTS packet,information which indicates the packet concerned is RTS is described; inthe Duration field, the time that lapses before the transmission andreception transaction for the packet concerned is completed (i.e. thetime until time T8) is described; in the RA field, the address of thedestination communication station (STA1) is described; and in the TAfield, the address of the transmitter station (STA0) itself isdescribed.

Here, attention should be paid to the following point: for STA0 todescribe in the Duration field the time that lapses before thetransaction according to the RTS/CTS procedure is completed, the timewhen the transaction is completed must be determined when RTS istransmitted. More specific description will be given. The transmittingSTA0 must determine all the transmission rate mode, including CTSpacket, Data packet, and ACK packet to be subsequently transmitted andreceived in the transaction in question, when it transmits RTS. Thetransmission rate mode determined here relates to the entiretransaction, and it is not permitted to set an individual transmissionrate mode with respect to each packet transmission within a transaction.

FIG. 19 illustrates the procedure according to which the transmittingSTA0 determines a transmission rate mode with respect to the entiretransaction concerned when it transmits RTS. When RTS is transmitted, itmust be determined where the time at which the transaction is completedis (T8 in FIG. 4). Therefore, the transmission rate mode is determinedonly due to information held by the transmitter station when ittransmits RTS (based on the amount of data communicated in the entiretransaction, the transmission rate, and the like). The transmission rateof each of the CTS, DATA, and ACK frames thereafter transmitted andreceived is basically in accordance with the rate applied in the RTS.That is, it is not permitted to set an individual transmission rate modewith respect to each packet transmission within a transaction.

Here, description will be given, referring to FIG. 18 again. The RTSpacket is received also by STA2 which is positioned in the vicinity ofSTA0. When STA2 receives the RTS signal, STA2 discovers the preamble andthus starts receive operation. Further, STA2 decodes PSDU based on theinformation obtained by decoding the PLCP header. Then, STA2 recognizesfrom the Frame Control field in the PSDU that the packet in question isRTS packet, and knows that STA0 desires to transmit information of somekind. Further, STA2 recognizes from the RA field that STA2 itself is notthe destination communication station. Then, STA2 does not monitor themedium and recognizes that the medium is occupied until the transactionin question is completed, and stops its own transmission so as not tohinder STA0's desire to send. To refer to this operation, it is saidthat a surrounding station “sets NAV (Network Allocation Vector).” TheNAV is effective during the period indicated in the Duration field, andSTA2 is kept in transmission prohibited state until time T8.

The RTS packet is received also by STA1 which is the intendeddestination. STA1 decodes the PSDU according to the same procedure asdescribed above, and thereby recognizes that STA0 desires to transmitpackets to STA1 itself. Then, STA1 waits for a SIFS (described above)and returns a CTS packet at time T3.

At this time, the transmission rate mode of the CTS packet must beidentical with that of RTS. Further, in the Frame Control field of thePSDU, it is described that the packet concerned is CTS packet; in theDuration field, the time that elapses between the transaction concernedis completed (i.e. time until time T8) is described; and in the RAfield, the address of the destination communication station (STA1) isdescribed.

The CTS packet is received also by STA3 which is positioned in thevicinity of STA1 as the destination of transmission. STA1 decodes thePSDU according to the same procedure as described above, and recognizesthat “some nearby communication station is expected to receive packetsuntil time T8.” STA3 sets an NAV and refrains from its own transmissionuntil the transaction concerned is completed so as not to hinder STA1'sdesire to receive. The NAV is effective during the period indicated inthe Duration field, and STA3 is also kept in transmission prohibitedstate until time T8.

The CTS packet is received also by STA0 as the destination. STA0 decodesthe PSDU according to the same procedure as described above, andrecognizes that STA1 has got ready to receive. Then, STA0 waits for aSIFS, and starts the transmission of Data packet at time T5.

If the transmission of the Data packet is completed at time T6 and STA1decodes them without any error, STA1 waits for a SIFS and returns ACK attime T7. When STA0 receives it, the transmission and receptiontransaction equivalent to one packet is completed at time T8.

At time T8, STA2 and STA3 which are nearby communication stations putdown the NAV, and return to ordinary transmitting and receiving state.

In summary, in the above-mentioned RTS/CTS procedure, the followingsurrounding stations are prohibited from carrying out transmission: a“surrounding station for STA0 as the transmitter station,” or STA2,which could receive RTS, and a “surrounding station for STA1 as thereceiver station,” or STA3, which could receive CTS. Thus, informationtransmission from STA0 to STA1 and a return of ACK are carried outwithout being hindered by a sudden transmit signal from surroundingstations. Therefore, the communication quality is maintained.

FIG. 20 illustrates an example of an operation sequence followed whenthe above-mentioned packet transmission and reception transaction basedon the RTS/CTS procedure in IEEE802.11 is implemented with trafficthrough TCP/IP (Transmission Control Protocol/Internet Protocol). TheTCP/IP is a typical communication protocol for transmission control andpath control.

Even if communication is of one way in terms of application, ACK isreturned by the TCP layer typically every two segments, and it isbrought into the form of asymmetrical two-way communication in the MAClayer. The ACK of the TCP layer is small in amount of data, and thusRTS/CTS is not used together. However, RTS/CTS is used together withrespect to data traffic. The figure illustrates such an example.

In the example illustrated in FIG. 20, packets equivalent to 24 times intotal are transmitted and received in the MAC layer to transmit fivesegments, Data0 to Data4. That is, while the operation in the TCP layeris relatively simple, complicated operation is performed in the MAClayer.

As described up to this point, problems of access contention andbandwidth assurance can be solved by implementing the transmitting andreceiving procedures based on CSMA in combination with the RTS/CTSmethod in accordance with IEEE802.11. Meanwhile, several problemsdescribed below are left unsolved.

(1) Imperfection of Transmission Rate

When data is transmitted and received according to the RTS/CTSprocedure, the transmission rate of data packets must be determinedbefore an RTS packet is transmitted.

This is because, for a transmitter station to describe in the Durationfield the time that lapses before a transaction based on the RTS/CTSprocedure is completed, the time at which the transaction is completedmust be determined when it transmits an RTS. This means that all thetransmission rate mode, including CTS packet, Data packet, and ACKpacket subsequently transmitted and received in that transaction must bedetermined when RTS is transmitted. The transmission rate modedetermined when RTS is transmitted relates to the entire transaction; itis not permitted to set an individual transmission rate mode withrespect to each packet transmission within a transaction. Therefore, thetransmission rate is imperfect.

Further, transmitter stations cannot grasp the state of reception inreceiver stations in real time. Therefore, determining the transmissionrate of the entire transaction when RTS is transmitted results in alower probability that data packets are transmitted at the optimaltransmission rate corresponding to the state of reception in thereceiver station.

An example will be taken. Patent Document 1 discloses a method forpacket transfer communication. The method is such that, when thereceiving end receives an RTS packet, it measures the environment andstate of communication at that point of time; then, the receiving endadds the result of the measurement to the CTS packet and returns it tothe transmitting end; thereby the transmitting end optimizes thecommunication rate and the like. However, this case is also nothing butanother case where the transmitting end determines the transmission rateof the entire transaction before it transmits an RTS packet. Therefore,the imperfection of transmission rate is not eliminated.

The RTS transmitter station is required to: determine the transmissionrate of the entire transaction including CTS packet, Data packet, andACK packet transmitted and received as the outcome of RTS transmission;determine a value to be written as the transaction finish time, that is,Duration based on a value obtained by dividing the amount of transmitteddata by the transmission rate; and describe the determined value in RTS.Patent Document 1 describes that the CTS transmitter station determinesa transmission rate through the reception of RTS; however it does notrefer to how to obtain the value in the Duration field described in RTSby the RTS transmitter station prior thereto.

If the CTS transmitting station sets a high transmission rate, thetransaction is completed ahead of the Duration determined when RTS istransmitted. As a result, the surrounding stations which heard the RTSstill keep a NAV set and refrain from their own transmission after thetransaction is completed, and this is a waste of bandwidth. If the CTStransmitting station sets a low transmission rate, the transaction isnot completed even after the Duration determined when RTS is transmittedhas passed. Nevertheless, surrounding stations which heard RTS put downthe NAV and start transmit operation, and this induces collision.

(2) Difficulty of Data Unit Multiplexing with RTS/CTS Used Together

In the conventionally known RTS/CTS procedure, the data transmitting enddetermines a data rate before it transmits an RTS. It does not giveconsideration to the following: the data receiving end determines a datarate and further transmits a plurality of data units as one data packet.To enhance the efficiency of the MAC layer (Refer to FIG. 20.), it ispreferably that the receiving end should be capable of determining adata rate and transmitting a plurality of data units as one data packet.However, the conventional RTS/CTS format cannot implement this.

(3) Concomitant Use of Delay Acknowledgement and RTS/CTS Procedure

The IEEE802.11 standard is premised on the immediate ACK wherein anacknowledgement (ACK) is returned as soon as data is received. To reducethe overhead of ACK packets, delay ACK can be used.

However, some problems arise. For example, if delay ACK is used togetherwith the RTS/CTS procedure, there is a possibility that an RTS istransmitted for the purpose of retransmission before an ACK cannot bereturned to the transmitting end. Thus, with the existing format, theconcomitant use of delay acknowledgement and the RTS/CTS procedure hasits limit in the enhancement of efficiency.

(4) Methodology For Selective Acknowledgement

It is known that carrying out selective acknowledgement enhances theefficiency of the MAC layer. The IEEE802.11 standard does not giveconsideration to selective acknowledgement itself. The selectiveacknowledgement referred to here is a retransmission control method. Inthis method, ACK or NACK information is fed back to the transmitting endwith respect to an arbitrary packet which was or was not successfullyreceived. Thereby, retransmission is carried out only with respect topackets which the receiving end could not receive.

Implementation of selective acknowledgement requires massive memory andits processing places a significant burden; therefore, it has not beactually applied so much. However, necessity for selectiveacknowledgement will be more and more increased in the future. With thisbeing the situation, the following case is expected: communicationstations in the initial stage do not have selective acknowledgementimplemented but communication stations developed later have selectiveacknowledgement implemented. In this case, it is unfavorable that bothcannot communicate with each other, and a format wherein downwardcompatibility is maintained is desired.

Further, up to now consideration has not given to concomitant use ofselective acknowledgement, a technique for the receiving end todetermine a data rate, and the RTS/CTS procedure. Therefore, this cannotbe implemented by the existing format.

(5) Influence of Overhead Produced in MAC Layer

The IEEE802.11 standards independently defines RTS frame, CTS frame, ACKframe, and the like. (Refer to FIG. 16.) For this reason, if individualinformation is to be transmitted, they must be respectively transmittedby separate frames.

However, each time a frame is transmitted, overheads, such as preamble,are produced. In particular, if the transmission rate is high, theamount of overheads becomes too large to neglect.

Needless to add, increase in overheads in the MAC layer is undesirablebecause it contributes in such a direction as to limit the bandwidthwhich can be provided for the upper level layers, such as TCP/IP.

[Patent Document 1] Japanese Unexamined Patent Publication No.2000-151639

[Non-Patent Document 1] International Standard ISO/IEC 8802-11: 1999(E)ANSI/IEEE Std 802.11, 1999 Edition, Part11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications

[Non-Patent Document 2] ETSI Standard ETSI TS 101 761-1 V1.3.1 BroadbandRadio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC)Layer; Part1: Basic Data Transport Functions

[Non-Patent Document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio AccessNetworks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2:Radio Link Control (RLC) sublayer

[Non-Patent Document4] C. K. Tho, “Ad Hoc Mobile Wireless Network”(Prentice Hall PTR)

SUMMARY OF THE INVENTION

An object of the present invention is to provide an excellent wirelesscommunication system, wireless communication device and wirelesscommunication method, and computer program wherein the RTS/CTS method isused together and yet access control based on CSMA can be favorablycarried out.

Another object of the present invention to provide an excellent wirelesscommunication system, wireless communication device and wirelesscommunication method, and computer program wherein a plurality of typesof frames, such as RTS, CTS, DATA, and ACK, are multiplexed and therebyflexile transmitting and receiving procedures can be provided andoverheads can be reduced.

The present invention has been made with the above problems taken intoaccount. A first aspect of the present invention is a wirelesscommunication system wherein random access is carried out using togetherthe RTS/CTS method in which a communication station as the origin ofdata transmission transmits a request to send packet RTS, and startsdata transmission in response to the reception of a response packet CTSfrom a communication station as the destination of data transmission.

The RTS receiving communication station measures the quality of areceive signal, determines a data rate based on the result of themeasurement, describes the data rate in CTS, and transmits the CTS.

The CTS receiving communication station carries out data transmission asthe outcome of the reception of the CTS information based on the datarate described in the CTS.

The “system” described here refers to what is obtained by logicallyaggregating a plurality of devices (or functional modules whichimplement specific functions). It does not matter whether the devices orfunctional modules are placed in a single enclosure or not.

The RTS transmitting communication station describes in RTS the timethat lapses before the reception of a packet, including CTS information,transmitted as the outcome of the reception of RTS information iscompleted, as medium reservation time information Duration. The CTStransmitting communication station describes in CTS the time that lapsesbefore the reception of a data packet transmitted as the outcome of thereception of CTS information is completed, as medium reservation timeinformation Duration. Other communication stations that receive RTS orCTS set a NAV (Network Allocation Vector), and keep it in effect for themedium reservation time information Duration and bring themselves intransmission prohibited state.

In a wireless communication system according to the first aspect of thepresent invention, a transmission rate can be selected based on thequality of receive signal. Therefore, the imperfection of transmissionrate can be wiped off.

Here, the following constitution may be adopted: the RTS sendingcommunication station describes in RTS factor information referred to bythe RTS receiving communication station when it determines a data rate.The RTS receiving communication station determines a data rate withconsideration given to the factor information described in the RTS, inaddition to the result of measurement of the quality of receive signal.

As the factor information described here, information related to whethera high data rate should be aggressively determined or a low data rateshould be passively determined is described. Further, the followingconstitution may be adopted: the RTS transmitting communication stationmeasures the transmission error rate for the RTS receiving communicationstation. Then, based on the result of the measurement, factorinformation (Rate Strategy) is determined. Or, factor information may bedetermined based on the difference in transmission power between RTSpacket and data packet, the causal relationship between data rate andtransmission power, or the like.

In these cases, a data rate is determined according to the quality ofRTS receive signal at the receiving end. A transmission rate isdetermined with consideration additionally given to information, such aserror rate monitored by the transmitting end. Therefore, thecorrespondence between reception quality and error rate arising fromfluctuation in channel state or fluctuation in the amount ofinterference can be corrected in two steps.

Further, the following constitution may be adopted: the RTS transmittingcommunication station describes in RTS information related to one ormore data units the station tries to transmit. The RTS receivingcommunication station determines medium reservation time informationDuration in which the reception of all the data packets is completed.The medium reservation time information Duration is determined based onthe information related to the data units, described in the RTS and thedetermined data rate. Then, the station describes it in CTS. In thiscase the data transmitting communication station which receives the CTScarries out data transmission as the outcome of the reception of CTSinformation. The data transmission is carried out based on the data ratedescribed in the CTS so that the data transmission will be completed ina time specified by the medium reservation time information.

Examples of the information related to data units, referred to here,include: the data length of each data unit for which transmission istried; the sum of data lengths of one or more data units for whichtransmission is tried; the length of time required for transmitting oneor more data units for which transmission is tried; information on thenumber of data units for which transmission is tried (in this case,however, data units are of fixed length); and the like.

In these cases, the RTS/CTS procedure can be used together and furtherdata units can be effectively multiplexed. Further, the RTS/CTSprocedure and delay ACK can be implemented. As a result, the amount ofoverheads produced in the MAC layer can be reduced.

In this case, when the RTS receiving communication station determinesmedium reservation time information after the reception of RTS, thestation takes into account the time at which the station itselfcompletes data reception. Or, when the RTS receiving communicationstation determines medium reservation time information after thereception of RTS, it gives consideration so that the period informationdescribed in the RTS is not exceeded.

For example, the following constitution may be adopted: the RTStransmitting communication station assigns sequence numbers to transmitdata units, and describes in RTS the first sequence number of data thestation tries to transmit. Thereby, the station notifies of informationrelated to the data units it tries to transmit. Meanwhile, the RTSreceiving communication station refers to the first sequence number ofdata unit, described in the RTS, and extracts and excludes data unitsthe station has already received from data units for which transmissionis tried. Then, the station determines medium reservation timeinformation.

Further, if selective acknowledgement is applied, the followingconstitution may be adopted: the RTS transmitting communication stationdescribes reception acknowledgement information in RTS. The receptionacknowledgement information includes the first sequence number of dataunits for which data transmission is tried and bit map informationobtained by mapping the reception confirmation information on thesubsequent data units into bits corresponding to the relative positionsto the first sequence number. Further, the station generates informationrelated to the data units with only data units for which receptionconfirmation has not been obtained taken as objects to be transmitted.Meanwhile, the RTS receiving communication station refers to the bit mapinformation described in the RTS. The station extracts data units thestation has already received from data units for which transmission istried, and excludes them from the objects to be transmitted. Then, thestation determines medium reservation time information.

Further, the following constitution may be adopted: if data units theRTS receiving communication station has already received are included indata units as objects to be transmitted, described in RTS, the stationdescribes ACK information in CTS. In this case, the CTS receivingcommunication station excludes data units the RTS receivingcommunication station has already received from objects to betransmitted based on the ACK information added to the CTS. Then, thestation carries out data transmission based on the data rate describedin the CTS.

Further, the following constitution may be adopted: the RTS receivingcommunication station stores one or more pieces of data lengthinformation of data units taken as objects to be transmitted, of datafor which transmission is tried as the outcome of the RTS. Then, thestation describes in CTS the sequence number of the last data unitstored. In this case, the CTS receiving communication station stores thesequence number of the last data unit described in the CTS. When an RTSis transmitted next time, the station generates information related todata units for which transmission is tried in such form that the dataunit indicated by the sequence number of the last data unit is notincluded in objects to be transmitted.

A second aspect of the present invention is a wireless communicationsystem wherein a plurality of data units are transmitted and received.Sequence numbers are assigned to transmit data units to identify thedata units. At the same time, if selective acknowledgement is applied,the following is caused to take place:

The receiving communication station describes the following in receptionacknowledgement information ACK: the sequence numbers of data unitscompletely received in the order of sequence number and the bit mapinformation represented by mapping the state of reception of data of thesubsequent sequence numbers into relative bit positions from thesequence numbers. The station transmits the reception acknowledgementinformation ACK.

The transmitting and receiving communication stations add informationindicating a causal data flow to transmit data and acknowledgement.

In a wireless communication system according to the second aspect of thepresent invention, information required for selective acknowledgement ismanaged on a data flow-by-data flow basis. Such information includes thesequence numbers of completely received data units, bit map informationwherein the state of reception of the subsequent data units aredescribed, and the like. Thus, a plurality of service classes can behandled by different sequence numbers. Therefore, communication can bemaintained regardless of whether selective acknowledgement isimplemented or not.

Some receiver stations do not support selective acknowledgement. Suchreceiver stations transmit the bit map information with zero set for allthe bits of the bit map field. Thus, the receiver stations can ensurecommunication with transmitter stations which support the selectiveacknowledgement function.

Further, some transmitter stations do not support selectiveacknowledgement. Such transmitter stations disregards the bit mapinformation transmitted from the receiving end, and transmits the bitmap information to be transmitted with zero set for all the bits of thebit map field. Thus, the transmitter stations can ensure communicationwith receiver stations which support the selective acknowledgementfunction.

Further, with respect to at least one data flow, the ARQ system whereinsequence numbers are not assigned is adopted. When Link session isestablished, data is transmitted and received through the data flow, andthereby the sequence numbers of the transmitter station and the receiverstation are synchronized with each other.

A third aspect of the present invention is a wireless communicationsystem wherein random access is carried out using the RTS/CTS methodtogether. The RTS/CTS method is such that: the communication station asthe origin of data transmission transmits a request to send packet RTS.In response to the reception of a response packet CTS from thecommunication station as the destination of data transmission, thecommunication station as the origin of data transmission starts datatransmission.

The transmission and reception of packets wherein any two or more ofrequest to send RTS, confirmative advice CTS, data, and acknowledgementACK are multiplexed are permitted.

The amount of overheads produced in the MAC layer can be significantlyreduced by including pieces of information, such as RTS, CTS, DATA, andACK, having different purposes in one packet.

In this case, the communication station which receives a packetcontaining RTS information transmits a packet containing CTS informationaccording to the contents of the RTS. The communication station whichreceives a packet containing CTS information transmits a packetcontaining data according to the contents of the CTS.

Here, the following constitution may be adopted: the RTS transmittingcommunication station can specify whether the receiving communicationstation may add RTS information to CTS or not. In this case, in RTS,first information which indicates whether RTS information may be addedto CTS transmitted as the outcome of the RTS or not is described. Then,the RTS receiving communication station judges whether it may add RTSinformation to CTS based on the first information in the RTS. If thereis data the station desires to transmit to the RTS transmittingcommunication station, it adds RTS information for the purpose oftransmitting the data to CTS, and transmits the CTS.

In this case, the RTS transmitting communication station determines thefirst information, for example, based on information elements containedin the packets received in the past from the station to which the RTS isdirected. The information element described here refers to More bitwhich indicates the presence of subsequent transmit data in the packet,and the like. Alternatively, it may be judged from other means thaninformation elements that subsequent transmit data is present in thestation to which the RTS is directed to determine the first information.Examples include the following case: a station receives an RTS from astation to which RTS is directed. However, the receiving station itselfreceived a packet from another surrounding station and thus has set aNAV. Therefore, the station cannot return a CTS, and the datatransmission sequence is stopped. There are also cases where it can bejudged based on any other communication history that subsequent transmitdata is present in the station to which RTS is directed.

Further, the following constitution may be adopted: the RTS receivingcommunication station recognizes from the information in the RTS thatthe RTS transmitting communication station is trying to transmit datathe receiving communication station itself has already received. In thiscase, the RTS receiving communication station adds ACK information toCTS to notify of the state of data reception.

Further, the following constitution may be adopted: in the datatransmitting and receiving procedure, the data transmittingcommunication station specifies whether the data receiving communicationstation should return an ACK. The ACK includes immediate ACK and delayACK. In this case, the following constitution may be adopted: the datareceiving communication station receives data requesting ACK, from thedata transmitting communication station. If the data receivingcommunication station judges that it has not transmitted an ACK of thedata, it adds ACK information when it transmits a packet of some kind tothe data transmitting communication station. In some cases, ACKinformation may be added at the transmitter station's request. At thistime, if the transmitter station judges that it has not received ACKinformation related to previously transmitted data, it sends a requestto add ACK information. However, with respect to packets in which RTSinformation is solely transmitted, ACK information need not be added byway of exception.

Further, data requesting an ACK may be transmitted as a packet directedto two or more communication stations.

Further, the following constitution may be adopted: the RTS transmittingcommunication station requests the RTS receiving communication stationto return an ACK of data which has been transmitted so far. In thiscase, the RTS transmitting communication station multiplexes an ACKrequest for requesting a return of ACK into an RTS for starting the nextdata transmission transaction, and transmits it.

Even if the RTS transmitting communication station requests an ACK, theACK is not returned sometimes. Possible causes include that transmitdata was not properly received, that data was property received but thereception of ACK failed, and the like. In these cases, the RTStransmitting communication station must verify whether to retransmitdata. In these cases, the RTS transmitting communication station maymultiplex an ACK request requesting a return of ACK into an RTS forstarting data retransmission transaction, and transmit it.

The communication station which received an RTS with an ACK requestmultiplexed returns an ACK which indicates whether it has properlycompleted the reception of data transmitted from the RTS transmittingcommunication station. If the station has failed to receive datatransmitted from the RTS transmitting communication station, it maymultiplex an ACK indicating that it failed to properly receive the data.Then, it may return a CTS requesting to retransmit the data.

A fourth aspect of the present invention is a wireless communicationsystem wherein the data transmitting communication station acquirestransmission right on the medium and carries out data communicationbetween it and the data receiving communication station.

After data transmission from the data transmitting communication stationis stopped, the presence or absence of subsequent data at the datatransmitting communication station is detected. If the subsequent datais present, it is made easier for the data transmitting communicationstation to acquire transmission right for transmitting the subsequentdata.

The state in which, though the subsequent data is present at the datatransmitting communication station, data transmission is stoppedcorresponds to the following cases, for example: cases where an RTS fromthe data transmitting communication station reaches the data receivingcommunication station and the data receiving communication stationreturns a CTS, but the data transmitting communication station does notstart data transmission; and cases where transmit data does not reachthe data receiving communication station.

The data receiving communication station can detect the presence/absenceof the subsequent data at the data transmitting communication stationbased on the communication history of transactions based on RTS/CTS. Or,the data transmitting communication station can include in a packet aspecific information element indicating the presence/absence of thesubsequent data and transmit the packet. In this case, the datareceiving communication station checks the information element, and canthereby detect the presence/absence of the subsequent data at the datatransmitting communication station.

According to CSMA, it is confirmed only for a predetermined period thatthe medium is clear, and then the operation waits only for an arbitrarybackoff time before transmission right is acquired. In the communicationenvironment wherein medium access control is carried out based on CSMA,the data receiving communication station tries to acquire transmissionright on the medium, and transmits a signal onto the medium foreliminating other communication stations' transmission right. Thereby,it can be made easier for the data transmitting communication station toacquire transmission right for transmitting the subsequent data.

In medium access control with the RTS/CTS method used together, thecommunication station as the origin of data transmission transmits arequest to send packet RTS. In response to the reception of a responsepacket CTS from the communication station as the destination of datatransmission, it starts data transmission. In case this medium accesscontrol is carried out, the data receiving communication stationtransmits a dummy RTS onto the medium for eliminating othercommunication stations' transmission right. Thereby, it can be madeeasier for the data transmitting communication station to transmit anRTS for transmitting the subsequent data. For example, when the datatransmitting communication station receives a dummy RTS, it can transmitan RTS with a shorter frame space for transmitting the subsequent data.

A fifth aspect of the present invention is a computer program written ina computer-readable format for performing on a computer systemprocessing for controlling communicating operation in the communicationenvironment wherein random access is carried out with the RTS/CTS methodused together. In the RTS/CTS method, the communication station as theorigin of data transmission transmits a request to send packet RTS; inresponse to the reception of a response packet CTS from thecommunication station as the destination of data transmission, it startsdata transmission. The program comprises:

a communication quality measuring step in which, when an RTS isreceived, the quality of receive signal is measured;

a data rate determining step in which a data rate is determined based onthe measured quality of receive signal;

a CTS transmitting step in which the data rate is described in CTS asthe outcome of the received RTS, and the CTS is transmitted; and

a data transmitting step in which, when CTS information is received,data transmission as the outcome of the CTS information is carried outbased on the data rate described in the CTS.

A sixth aspect of the present invention is a computer program written ina computer-readable format for carrying out on a computer system controlof communicating operation for transmitting and receiving a plurality ofdata units. Sequence numbers are assigned to transmit data units toidentify the data units. If selective acknowledgement is applied, thefollowing steps are provided:

a step in which, in response to data reception, the sequence numbers ofcompletely received data units in the order of sequence number, and bitmap information represented by mapping the state of reception of data ofthe subsequent sequence numbers into relative bit positions from thesequence numbers, are described in reception acknowledgement informationACK and are transmitted; and

a step in which information indicating a causal data flow is added, andtransmit data and an acknowledgement are transmitted.

A seventh aspect of the present invention is a computer program writtenin a computer-readable format for performing on a computer systemprocessing for controlling communicating operation in the communicationenvironment wherein random access is carried out with the RTS/CTS methodused together. In the RTS/CTS method, the communication station as theorigin of data transmission transmits a request to send packet RTS; inresponse to the reception of a response packet CTS from thecommunication station as the destination of data transmission, it startsdata transmission. The program comprises:

an information generating step in which varied information, includingrequest to send RTS, confirmative advice CTS, data, and acknowledgementACK, is generated; and

a packet transfer communication step in which a packet wherein any twoor more pieces of information of request to send RTS, confirmativeadvice CTS, data, and acknowledgement ACK are multiplexed is transmittedand received.

An eighth aspect of the present invention is a computer program writtenin a computer-readable format for performing on a computer systemprocessing for controlling the communicating operation in the followingcommunication environment: in the communication environment, thecommunication station acquires transmission right on the medium beforeit carries out data communication. The program comprises:

a step in which the presence/absence of transmit data from othercommunication stations is detected; and

a medium access control step in which, in response to the detection ofthe presence of transmit data from other communication stations, asignal for eliminating the other communication stations' transmissionright is transmitted onto the medium so as to make easier for therelevant communication station to acquire transmission right fortransmitting data.

The computer programs according to the fifth to eighth aspects of thepresent invention are computer programs written in a computer-readableformat for performing predetermined processing on a computer system bydefinition. In other words, when installed on a computer system, thecomputer programs according to the fifth to eighth aspects of thepresent invention deliver synergistic action on the computer system andoperate as wireless communication devices. When a plurality of suchwireless communication devices are started to build a wireless network,the same action and effect as with the wireless communication systemsaccording to the first to fourth aspects of the present invention can beobtained.

According to the present invention, an excellent wireless communicationsystem, wireless communication device and wireless communication method,and computer program wherein the RTS/CTS method is used together andfurther access control can be favorably carried out based on CSMA can beprovided.

Further, according to the present invention, an excellent wirelesscommunication system, wireless communication device and wirelesscommunication method, and computer program wherein a plurality of typesof frames, such as RTS, CTS, DATA, and ACK, are multiplexed, and therebyflexible transmitting and receiving procedures can be provided andoverheads can be reduced can be provided.

According to the present invention, a transmission rate can be selectedbased on the quality of receive signals. Therefore, the imperfection oftransmission rate can be eliminated. In addition, when a transmissionrate is determined, information, such as the error rate monitored by thetransmitting end, is also taken into account. Therefore, thecorrespondence between reception quality and error rate arising fromfluctuation in channel state or fluctuation in the amount ofinterference can be corrected in two steps.

Further, according to the present invention, data units can beeffectively multiplexed while the RTS/CTS procedure is used together.Further, the RTS/CTS procedure and delay ACK can be implemented. As aresult, the amount of overheads produced in the MAC layer can bereduced.

Further, according to the present invention, communication can bemaintained regardless of whether selective acknowledgement isimplemented or not.

Further, according to the present invention, the amount of overheadsproduced in the MAC layer can be significantly reduced by includingpieces of information, such as RTS, CTS, DATA, and ACK, having differentpurposes in one packet.

Other objects, features, and advantages of the present invention will beapparent from the embodiments of the present invention described laterand the more detailed description taken in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating an example of the disposition ofcommunication devices constituting a wireless communication system in anembodiment of the present invention.

FIG. 2 is a drawing schematically illustrating the functionalconstitution of a wireless communication device which operates as acommunication station in a wireless network in an embodiment of thepresent invention.

FIG. 3 is a drawing illustrating an example of a frame format used in awireless communication system according to the present invention.

FIG. 4 is a drawing illustrating an example of the constitution of PSDU.

FIG. 5 is a drawing illustrating several examples of PSDUs which can bedefined.

FIG. 6 is a drawing explaining the process for determining atransmission rate in a wireless communication system according to thepresent invention.

FIG. 7 is a drawing illustrating an example of the operation sequence ofthe RTS/CTS procedure according to the present invention.

FIG. 8 is a drawing illustrating an example of the application oftransmitting and receiving procedures according to the presentinvention.

FIG. 9 is a drawing illustrating another example of the application oftransmitting and receiving procedures according to the presentinvention.

FIG. 10 is a drawing illustrating a further example of the applicationof transmitting and receiving procedures according to the presentinvention.

FIG. 11 is a drawing illustrating the acknowledgement procedure relatedto data other than data flow 0 assumed in the present invention.

FIG. 12 is a drawing illustrating a concrete example of the interactionof individual fields which occurs when selective acknowledgement iscarried out with RTS/CTS used together.

FIG. 13 is a drawing illustrating an example of data transmitting andreceiving procedures taken when delay ACK and the RTS/CTS procedure areapplied.

FIG. 14 is a drawing illustrating an example of the sequence in whichtraffic is transmitted and received through TCP/IP in accordance withthe MAC procedure according to the present invention.

FIG. 15 is a drawing illustrating the frame format according toIEEE802.11a.

FIG. 16 is a drawing illustrating the constitution of frame fields inPSDU.

FIG. 17 is a drawing schematically illustrating an example of theoperation of the RTS/CTS procedure.

FIG. 18 is a drawing explaining the action which can occur atsurrounding stations when the RTS/CTS procedure is carried out betweenthe transmitter station and the receiver station.

FIG. 19 is a drawing explaining the procedure for determining thetransmission rate mode of the entire transaction when STA0 as the originof transmission transmits an RTS.

FIG. 20 is a drawing illustrating an example of the operation sequencein which packet transmission and reception transaction based on theRTS/CTS procedure according to IEEE802.11 is implemented with trafficthrough TCP/IP.

FIG. 21 is a drawing explaining an example of the application oftransmitting and receiving procedures according to the presentinvention.

FIG. 22 is a drawing explaining another example of the application oftransmitting and receiving procedures according to the presentinvention.

FIG. 23 is a drawing explaining a further example of transmitting andreceiving procedures according to the present invention.

FIG. 24 is a drawing explaining the mechanism for controlling thejudgment of whether “information indicating that an RTS may bemultiplexed into a CTS” should be set or not.

FIG. 25 is another drawing explaining the mechanism for controlling thejudgment of whether “information indicating that an RTS may bemultiplexed into a CTS” should be set or not.

FIG. 26 is a further drawing explaining the mechanism for controllingthe judgment of whether “information indicating that an RTS may bemultiplexed into a CTS” should be set or not.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, the embodiments of the present invention willbe described in detail below.

A. System Configuration

The propagation path for communication the present invention assumes iswireless, and a network is built among a plurality of communicationstations. The communication the present invention assumes is store andforward traffic, in which information is transferred on apacket-by-packet basis. With respect to each communication station, thefollowing description assumes single-channel station; however, thepresent invention can be expanded to cases where a transmission mediumcomprising a plurality of frequency channels, that is, multiple channelsis used.

In a wireless network according to the present invention, eachcommunication station can directly (randomly) transmit information inaccordance with an access procedure based on CSMA (Carrier SenseMultiple Access). Thus, the communication stations can build anautonomous and decentralized wireless network.

The autonomous and decentralized wireless communication system does nothave the relation of control station and controlled station. In such awireless communication system, for example, each communication stationsends beacon information. The station thereby informs othercommunication station located nearby (i.e. within the range ofcommunication) of the presence of itself, and informs them of theconstitution of the network. A communication station which newly emergesin some communication station's range of communication receives a beaconsignal, and thereby detects that it has entered the range ofcommunication. Further, the station decodes the information described inthe beacon, and can thereby recognize the network and participate in thenetwork.

In the wireless network according to the present invention,communication stations exchange beacon signals, and modestly andtemporally synchronize with each other. Thus, transmission control iscarried out wherein channel resources are effectively utilized bytransmission (MAC) frames having the time division multiple accessstructure. Therefore, each communication station can implement an accessmethod based on temporal synchronization, such as reserving a bandwidthand setting a preferential period of use.

The processing at each communication station, described below, isbasically carried out at every communication station that participatesin the network. In some cases, however, the processing described belowis not necessarily performed by all the communication stationsconstituting the network.

FIG. 1 illustrates an example of the disposition of communicationdevices which constitute the wireless communication system in anembodiment of the present invention. This wireless communication systemdoes not have the relation of control station and controlled station. Inthis communication system, each communication device autonomously anddecentralizedly operates, and an ad-hoc network is formed. The figureshows how the communication device #0 to the communication device #6 aredistributed in the same space.

In the figure, each communication device's range of communication isindicated by broken line. Each communication device can communicate withother communication devices located within its range, and the range ofcommunication is defined as a range in which a signal transmitted byeach station interferes with other communication devices. More specificdescription will be given. The communication device #0 is located in arange in which it can communicate with the communication devices #1 and#4 located nearby; the communication device #1 is located in a range inwhich it can communicate with the communication devices #0, #2, and #4located nearby; the communication device #2 is located in a range inwhich it can communicate with the communication devices #1, #3, and #6located nearby; the communication device #3 is located in a range inwhich it can communicate with the communication device #2 locatednearby; the communication device #4 is located in a range in which itcan communicate with the communication devices #0, #1, and #5 locatednearby; the communication device #5 is located in a range in which itcan communicate with the communication device #4 located nearby; and thecommunication device #6 is located in a range in which it cancommunicate with the communication device #2 located nearby.

When communication is carried out between certain communication devices,a “hidden terminal” exists. Hidden terminal is defined as acommunication device which can be heard by either of the communicationdevices communicating with each other but cannot by the other.

The scope of the present invention is not limited to the above ad-hocenvironment. It can be widely applied to other modes of communicationwherein each communication station can directly and asynchronouslytransmit information in accordance with the access procedure based onCSMA with the RTS/CTS procedure used together.

FIG. 2 schematically illustrates the functional constitution of awireless communication device which operates as a communication stationin a wireless network in an embodiment of the present invention. Thewireless communication device in the figure can carry out access controlbased on CSMA using the RTS/CTS procedure together and form a network inthe autonomous and decentralized communication environment in which acontrol station is not provided.

As illustrated in the figure, the wireless communication device 100comprises an interface 101, a data buffer 102, a central control unit103, a beacon generating unit 104, a radio transmitter unit 106, atiming control unit 107, an antenna 109, a radio receiver unit 110, abeacon analyzing unit 112, and an information storing unit 113.

The interface 101 exchanges varied information between it and externalequipment (e.g. personal computer (not shown)) connected to thiswireless communication device 100.

The data buffer 102 is used to temporarily store data transmitted fromconnected equipment through the interface 101 or data received through awireless transmission channel before the data is sent out through theinterface 101.

The central control unit 103 manages a series of informationtransmission and reception processing in the wireless communicationdevice 100 and controls accesses to the transmission channel in acentralized manner. In the central control unit 103, for example, accesscontrol is carried out based on CSMA with the RTS/CTS procedure usedtogether. In this embodiment, transmitting and receiving procedureswherein a plurality of types of frames, such as RTS, CTS, DATA, and ACK,are multiplexed are implemented. These procedures will be described indetail below.

The beacon generating unit 104 generates beacon signals periodicallyexchanged between the wireless communication device 100 and nearbywireless communication devices.

The radio transmitter unit 106 includes: a modulator which modulatestransmit signals by a predetermined modulation method; a D-A converterwhich converts digital transmit signals into analog signals; an upconverter which up converts analog transmit signals by frequencyconversion; a power amplifier (PA) which amplifies the power of upconverted transmit signals; and the like. (None of these items is shownin the figure.). The radio transmitter unit 106 wirelessly transmitsdata temporarily stored in the data buffer 102 and beacon signals at apredetermined transmission rate.

The radio receiver unit 110 comprises: a low noise amplifier (LNA) whichvoltage amplifies signals received from other stations through theantenna 109; a down converter which down converts voltage amplifiedreceive signals by frequency conversion; an automatic gain controller(AGC); an A-D converter which converts analog receive signals intodigital signals; a synchronization circuit for obtaining synchronism, achannel estimation circuit, and a demodulator which carries outdemodulation by a predetermined demodulation method; and the like. (Noneof these items is shown in the figure.) The radio receiver unit 110receives signals, such as information and beacons, transmitted fromother wireless communication devices during predetermined periods.

As radio transmission and reception methods for the radio transmitterunit 106 and the radio receiver unit 110, for example, varioustransmission methods which are applicable to wireless LANs and suitablefor relatively short distance communication can be applied.Specifically, the UWB (Ultra Wide Band) method, OFDM (OrthogonalFrequency Division Multiplexing) method, CDMA (Code Division MultipleAccess) method, and the like can be adopted.

The antenna 109 wirelessly transmits signals directed to other wirelesscommunication devices on a predetermined frequency channel. Or, theantenna 109 gathers signals sent from other wireless communicationdevices. In this embodiment, the wireless communication device 100 isprovided with a single antenna and is incapable of carrying outtransmission and reception in parallel.

The timing control unit 107 controls timing with which radio signals aretransmitted and received. For example, it controls the following timing:transmission timing and reception timing for various packets of RTS,CTS, DATA, ACK, and the like and packets in which these are multiplexed;timing with which the wireless communication device 100 transmitsbeacons; timing with which beacons are received from other stations; andthe like.

The beacon analyzing unit 112 analyzes beacon signals which could bereceived from neighbor stations, and analyzes the presence of nearbywireless communication devices and the like. For example, suchinformation as reception timing for beacons of neighbor stations andnearby beacon reception timing, is stored as nearby device informationin the information storing unit 113.

The information storing unit 113 stores the following: executionprocedure instructions (programs wherein the collision avoidanceprocedure and the like are described) for a series of access controloperation and the like, performed in the central control unit 103;nearby device information obtained from the result of analyses ofreceived beacons; and the like.

B. Frame Format (Packet Format)

FIG. 3 illustrates an example of the constitution of frame format usedin a wireless communication system according to the present invention.However, in this figure, the preamble, PLCP header, and the likeillustrated in FIG. 15 are omitted, and only the portion represented asPSDU (PHY Service Data Unit) is selected and shown.

As illustrated in the figure, the PSDU comprises MAC header portion andMSDU (MAC Service Data Unit: one data unit passed from the upper levellayer) portion. In this embodiment, a plurality of MSDUs can be placedin one PSDU. The MAC header portion comprises common MAC header portion,sub MAC header portion, and HCS (Header Check Sequence) portion. As subMAC header, a plurality of sub MAC headers can be placed.

FIG. 4 illustrates an example of the constitution of PSDU in thisembodiment. These frame examples correspond to conventional RTS frame,CTS frame, ACK frame, and data frame, respectively. (Refer to FIG. 16.)

The header length (HLen), destination address (RA), transmission originaddress (TA), and Duration defined in each frame in common correspond tothe common MAC header portion. Table 2 indicates the fields in thecommon MAC header portion in detail.

TABLE 2 Length Field Name [Bit] Description H LENGTH 1 MAC headerlength. Up to 255 bytes. Refer to this value to determine to what extentthe CRC computation for HCS should be carried out. RA 8 The destinationaddress of the relevant packet. If this is not matched with the relevantpacket's ID or broadcast ID, decoding is immediately stopped. TA 8 Thetransmission origin address of the relevant packet. Duration 2 Fieldindicating for what period of time in [μsec] reception is expected tolast after the completion of transmission of the relevant PHY burst.This is used for the receiver station to indicate a period for which itcan transmit and for surrounding other stations to set a NAV.

The sub MAC header portion is defined in different formats depending onthe purpose of each frame. For example, in a packet corresponding to RTSframe, RTS SMH (Sub MAC Header) is placed as sub MAC header; in a packetcorresponding to CTS frame, CTS SMH is placed as sub MAC header; in apacket corresponding to ACK frame, ACK SMH is placed as sub MAC header;and in a packet corresponding to data frame, DATA SMH is placed as subMAC header.

With respect to the RTS, CTS, and ACK frames, PSDU is constituted onlyof the MAC header portion, and the MSDU does not exist. To the dataframe, data specified by the DATA SMH is added as frame body.

FIG. 5 illustrates several examples of PSDUs which can be defined in thepresent invention. In the uppermost tier in FIG. 5, a packet in whichtwo MSDUs are multiplexed is shown. In the second tier, a packet inwhich one MSDU and RTS information are multiplexed is shown. In thethird tier, a packet in which CTS information and ACK information aremultiplexed is shown. In the lowermost tier, a packet in which one MSDU,RTS information, CTS information, and ACK information are multiplexed isshown.

If an RTS is to be transmitted, an RTS SMH is added as SMH; if a CTS isto be transmitted, a CTS SMH is added as SMH; if an ACK is transmitted,an ACK SMH is added as SMH; and if data units are to be transmitted,DATA SMHs are added by a number equivalent to the number of data unitsto be transmitted. Thus, pieces of information, such as RTS, CTS, DATA,and ACK, having different purposes can be multiplexed into one packet.

Next, the details of the components of each SMH will be describedreferring to Table 3 to Table 6. The leading field of each SMH is Typefield, in which an identifier indicating what information is containedin the relevant SMH is described. The length of SMH differs depending onthe type of SMH. Since the SMH has an intrinsic length with respect toeach Type, however, the receiving end can learn the length of therelevant SMH by referring to the Type field.

Fields in DATA SMH:

Table 3 illustrates the details of the fields in DATA SMH used whengeneral-purpose data, other than RTS, CTS, and ACK, is transmitted. Asillustrated below, the DATA SMH comprises Attribute, Sequence, andLength fields.

TABLE 3 Length Field Name [Bit] Description Type 1 Identifier indicatingthat this is an SMH for general-purpose data transmission. Attribute 1Identifier indicating the attribute of the data specified by the SMH.[2] Data Flow [2] ACK Type [1] More Bit [3] Fragment Sequence 1 Thesequence number of the data specified by the SMH. Rotation of 0 to 255is repeated. Length 2 Indicates the length in byte of the data specifiedby the SMH.

In the Attribute field, the identifier indicating the attribute of thedata specified by the relevant DATA SMH is described.

In the Data Flow portion, the data flow of the relevant data is placed.Here, it is assumed that data flows having a plurality of attributes ona link-by-link basis can be defined. Data flows having differentattributes are used to define different ARQ (Automatic Repeat reQuest)methods on a data flow-by-data flow basis, to handle traffic differentin priority from data flow to data flow, and to attain the like intendedpurposes.

In the ACK Type portion, information which indicates what ACK thereceiver that received the data should be requested to return is placed.Specifically, three types of information, immediate ACK (Im-ACK)request, delay ACK (Del-ACK) request, and ACK not required, arenotified. If the immediate ACK request is specified, the receiving endreturns a packet containing ACK information immediately after itreceives data. If the delay ACK request is specified, preparations forreturning ACK are made but the transmission of ACK information is notcarried out until a packet of some kind is transmitted to the origin oftransmission of data. (When a packet of some kind other than RTS istransmitted to the origin of transmission of data, an ACK SMH is addedto the packet and it is transmitted.) If ACK not required is notified,an ACK is not returned.

In the More Bit portion, information indicating whether, after therelevant packet is transmitted, more data to be transmitted has beenaccumulated or not is placed. The receiver stores the address of thepacket transmitting station at which the More Bit portion has been mostrecently set, and, in some cases, polls that station. The pollingprocedure is not related directly to the subject matter of the presentinvention, and thus further description will not be given.

If the relevant data is fragmented, the Fragment portion is used toindicate borders in the data.

In the Sequence field, the sequence number assigned to the relevant datais placed. It is assumed that sequence numbers 0, 1, 2, 3, . . . areassigned to data units in ascending order as is performed in ordinarydata communication. The sequence numbers of 0 to 255 are repeatedly usedby rotation.

In the Length field, the length of the relevant data is placed.

Fields in ACK SMH:

Table 4 illustrates the components of ACK SMH fields used when ACKinformation is transmitted. As described below, the ACK SMH comprisesACK Attribute, ACK Sequence/Type, and Received MAP fields.

TABLE 4 Length Field Name [Bit] Description Type 1 Identifier indicatingthat this is an ACK SMH. ACK Attribute 1 Identifier indicating theattribute of ACK. [2] Data Flow [6] Reserved ACK 1 Transcribes DATA Typeof the object of ACK Sequence/Type if DATA Flow is 0. Describes sequencenumber in the other cases. Received MAP 2 Used when Data Flow is not 0.ACK/NACK information for Selective ACK.

In the ACK Attribute field, the identifier indicating the attribute ofACK is described. The Data Flow portion indicates in which data flow thedata as the object of ACK information is transmitted. Here it is assumedthat, if data flow 0 is selected, immediate ACK (Im-ACK) is used, and ifany other data flow is selected, selective ACK (Sel-ACK) is used.

The ACK Sequence/Type field is a field for indicating up to whichsequence number of the relevant data flow has been sequentially receivedif the data flow is not 0. For example, the value of 6 is placed, thatindicates data of up to sequence number 6 has been all successfullyreceived. If 0 is selected for data flow, immediate ACK is used, and thedata type of that data is transcribed to indicate which data the ACK isfor.

The Received MAP field is a field for indicating whether any data thathas been received after the data indicated by the ACK Sequence exists ornot. Relative to the sequence number indicated by the ACK Sequence, thesubsequent sequence numbers are brought into correspondence by bit map,and bits corresponding to data (sequence number) which has been receivedare marked. The MSB corresponds to the number next to the sequencenumber indicated by the ACK Sequence. As an example, it is assumed that:the Received MAP field is constituted of eight bits; the sequence numberin the ACK Sequence field is 23; and the Received MAP is 00100000. Thismeans the following: data of up to #23 has been all received; (theReceived MAP represents the success or failure in receiving the eightpieces of data of #24 and the following numbers by bit map); thereception of data of #24 and #25 failed; data of #26 has been received;and the subsequent data has not been received.

The method of transmission and reception wherein data units received atintervals are held at the receiving end is designated as selectiveacknowledgement (Selective ACK). If the receiver supports selectiveacknowledgement, a Received MAP is generated in accordance with theabove-mentioned rule. If the receiver does not support selectiveacknowledgement, data which could not sequentially received isdiscarded, and zero is placed in all the bits of the Received MAP field.

Fields in RTS SMH:

Table 5 illustrates the components of RTS SMH fields used when RTSinformation is transmitted. As described below, the RTS SMH comprisesRTS Attribute, RTS Sequence, RTS Received MAP, Max Duration, and Length(or Number Of Data Unit) fields.

TABLE 5 Length Field Name [Bit] Description Type 1 Identifier indicatingthat this is a (Directed) RTS SMH. RTS 1 Identifier indicating theattribute Attribute of RTS. [2] Data Flow [2] Rate Strategy [1] Rate Set[1] RTS/CTS Mux [1] ACK Request [1] Reserved RTS Sequence 1 The sequencenumber of the leading MSDU to be transmitted. RTS Received MAP 2 Withrespect to MSDUs subsequent to RTS Sequence, the bit corresponding todata recognized to have been transmitted is marked with “1.” MaxDuration 1 The maximum data transmission time which can be allowed bythe trans- mitting end. Length 1 × N Information related to data unit tobe transmitted (value indicating the data length information). (Numberof 1 (For example, if the data units are Data Unit) of fixed length) thenumber of data units to be transmitted. (Directed 8 (Only in case ofDirected RTS) the Address) MAC address of the node as the object of RTS.

In the RTS Attribute field, the identifier indicating the attribute ofthe RTS information is described.

In the Data Flow portion, the data flow of data for which transmissionis about to be tried is placed.

In the Rate Strategy portion, auxiliary information is placed which isreferred to when the transmission rate of data is determined. Thisauxiliary information is used to bias criteria for determining: whetherto aggressively select a high rate for transmission rate; whether toslightly aggressively select a slightly high rate; whether to slightlypassively select a slightly low rate; or whether to passively select alow rate. The usage of the Rate Strategy portion will be described indetail later.

In the Rate Set portion, indication is placed which indicates whether tofix the transmission rate at the value applied in RTS for transmissionor not. If transmission is carried out with the transmission rate fixed,the entry in the Rate Strategy portion is disregarded.

In the RTS/CTS Mux portion, information is placed which indicateswhether to permit adding RTS information to a CTS packet transmitted inresponse to the relevant RTS or not. If the addition of RTS ispermitted, the station which receives the relevant RTS and returns a CTSoperates as follows: if the station holds data directed to the stationas the origin of transmission of the RTS, it is permitted to add RTSinformation to the CTS and transmit it. Thus, two-way communication iscarried out by one transaction.

In the ACK Request portion, information is placed which indicateswhether to request to add ACK information to a CTS packet transmitted inresponse to the relevant RTS or not. If the addition of ACK informationis requested, the station which receives the RTS and returns a CTSoperates as follow: the station adds the ACK information of data whichit has received from the station of the origin of transmission of theRTS to the CTS and transmits it.

The RTS Sequence field is a field in which the sequence number of theleading data of pieces of data for which the station as the origin oftransmission tries to transmit is described. The station as thedestination of RTS refers to this field. Thereby, the station can learnwhether the station as the origin of transmission is trying to transmitdata which is a duplicate of the data already received or not.

The RTS Received MAP field produces an effect when selectiveacknowledgement is used together. Relative to the sequence numberindicated in the RTS Sequence field, pieces of data of the subsequentsequence numbers are brought into correspondence by bit map. Then, thebits corresponding to data (sequence numbers) the reception of which thereceiver station recognizes to have been completed are marked. The MSBcorresponds to the sequence number indicated by RTS Sequence. As anexample, it is assumed that: the RTS Received MAP field is constitutedof eight bits; the sequence number in the RTS Sequence field is 23; andthe Received MAP is 00100000. This means the following: pieces of dataof #23, #24, and #25 are skipped, and for pieces of data of #26 and thefollowing numbers, transmission is continuously tried. The station asthe destination of RTS refers to this field, and can thereby learnwhether the station as the origin of transmission is trying to transmitdata which is a duplicate of the data already received.

The Max Duration field is used for the transmitting end to set themaximum period of data transmission in the present data transmissiontransaction. As an example, it is assumed that, with respect to thepresent transaction, data transmission is to be completed within 200microseconds on the grounds of the transmitter station. In this case, avalue indicating 200 microseconds (or a value obtained by subtractingthe time required for the RTS/CTS procedure from 200 microseconds) isplaced in this field. The station as the destination of RTS has right todetermine a period of data reception. At this time, the stationdetermines a period of data reception so that the period indicated inthe Max Duration field will not be exceeded.

In the Length field, the length of data for which transmission is triedis described. In case of a first example of constitution, a plurality ofpieces of Length information (e.g. number of bits and number of bytes)are defined if it is permitted to transmit one or more data units in onepacket. Here, it is assumed that, for example, N pieces of Lengthinformation are defined. (N is typically a fixed value but may be anon-fixed value.) Data units the reception of which the receiving endrecognizes to have been completed are excluded from data for whichtransmission is tried. As an example, it is assumed that: the sequencenumber in the RTS Sequence field is 23; and the Received MAP is00100000. In this case, the length of data unit of #23 is placed inLength0, and the length of data unit of #24 is placed in Length1. InLength2, the length of data unit of #26, not that of data unit of #25which has been already received, is placed.

As a second example of constitution, the sum of the lengths of aplurality of data units for which transmission is tried can be describedas Length information. In this case, in the RTS Sequence field, thesequence number of the leading data of a plurality of data units isdescribed.

As a third example of constitution, the length of data for whichtransmission is tried can be expressed in time and described in theLength field. In this case, in the Length field, a plurality of lengthsof time for which the channel is occupied when data or a plurality ofdata groups for which transmission is tried are transmitted aredescribed with respect to cases where they are transmitted at severaldata rates. Here, it is assumed that, for example, N pieces of Lengthinformation are defined. (On the assumption that transmission is carriedout at N different data rates, the length of time required for datatransmission is described.)

As a fourth example of constitution, the Number Of Data Unit field canbe defined in place of the Length field if the length of a data unitmainly transmitted is a fixed value. In this case, in the field, thenumber of data units for which transmission is tried is described.

In the RTS SMH, the Directed Address field can be defined in addition tothe foregoing. This is used in the following case: broadcast ormulticast is set in the destination (RA) field of the common MAC headerbut RTS information is to be transmitted to some specific station. Inthe Directed Address field, the address of the destination of RTSinformation is described.

Fields in CTS SMH:

Table 6 illustrates the components of CTS SMH fields used when CTSinformation is transmitted. As described below, the CTS SMH comprisesCTS Attribute, RATE, and Stored Sequence fields.

TABLE 6 Length Field Name [Bit] Description Type 1 Identifier indicatingthat this is a CTS SMH. CTS Attribute 1 Identifier indicating theattribute of CTS. [2] Data Flow [6] Reserved RATE 1 Specifies the datarate class of transmit data. Stored Sequence 1 Notifies that the Lengthinformation of up to the relevant Sequence number has been stored.

In the CTS Attribute field, the identifier indicating the attribute ofthe relevant CTS information is described.

In the Data Flow portion, the data flow of data for which transmissionis about to be tried is placed.

In the RATE field, a value which indicates the transmission rate of datatransmitted in response to the transmission of CTS is placed.

In the Stored Sequence field, the Sequence number is placed whichindicates up to what Sequence number the data receiving end (CTStransmitting end) has held data length information. If the relevantcommunication station is not provided with a function of holding datalength information, the number immediately preceding the Sequence numberindicated in the RTS Sequence field is placed. Or, a value is placedwhich represents NULL indicating that the relevant communication stationis not provided with a function of holding data length information.

In the example described above, CMH and SMH independently exist in PSDUand a plurality of different types of SMHs are independently added forconvenience of explanation. An aspect of the objects of the presentinvention is attained as long as the format contains the similarcomponents of information; therefore, the present invention is notlimited to the above-mentioned format.

An example will be taken. Information elements equivalent to the MACheader are preferably disposed at the head of a packet in a lump, asdescribed above. However, they can be disposed behind or between dataunits constituting a packet sometimes. In particular, the DATA SMH isdisposed immediately before the data unit to which the relevant SMHcorresponds in some examples of utilization.

In the examples described above, the RA, TA, and Duration fields aredisposed in CMH. However, these fields are disposed in the PLCP portionsin some examples of utilization.

C. Transmission Rate Determination Process

Referring to FIG. 6, description will be given to a process fordetermining a transmission rate in a wireless communication systemaccording to the present invention.

In the present invention, access control based on CSMA is carried outwith the RTS/CTS procedure used together. In such a case, CTSinformation is transmitted in response to the reception of RTSinformation, and data is transmitted in response to the reception ofCTS. The CTS transmitting station measures the quality of RTS receivesignal, and thereby determines a transmission rate at which receptioncan be implemented and notifies the station as the destination of CTS ofit. The station as the destination of CTS transmits data in response tothe CTS information. At this time, the station applies the transmissionrate indicated in the RATE field in the CTS as the transmission rate ofthis data. The process for determining the transmission rate is asfollows:

Step 1: the quality, such as reception SINR, is measured based on RTSreceive signal.Step 2: the quality or transmission rate class measured according to thevalue indicated in the Rate Strategy field in RTS is biased, and atransmission rate is determined. In case of IEEE802.11a, for example, itis determined from among eight different transmission rate modesprescribed: 6, 9, 12, 18, 24, 36, and 54 Mbps.

When the measured quality is biased, the measured reception SINR valueis biased at the level of several dB in accordance with the valueindicated by Rate Strategy. Based on the biased reception SINR value, atransmission rate class at which reception can be implemented isdetermined by lookup table or the like.

When the transmission rate class is biased, a transmission rate class atwhich reception can be implemented is extracted by lookup table or thelike based on the measured reception SINR value. The extractedtransmission rate class is shifted in accordance with the valueindicated by Rate Strategy.

This shift processing may be applied, for example, only when a high-rateclass is selected or only when a low-rate class is selected depending onthe characteristics of the physical layer.

The data transmitting station (RTS transmitting station) which receivedCTS holds the transmission rate class, specified in the RATE field, as adefault transmission rate class directed to the station as the origin ofCTS transmission. Thus, the station can refer to it when the stationtransmits an RTS or data next time. When the station transmits an RTS,it refers to this default transmission rate class, and applies thistransmission rate class or the next lower transmission rate class(higher by one level in tolerance to noise). When data is transmittedwithout using RTS/CTS together, the default transmission rate class isapplied in place of the transmission rate class notified by CTS.

When the RTS transmitting station transmits data units, it counts thefollowing with respect to each destination: the number of times when adata unit is transmitted for the first time, and the number of timeswhen a data unit is retransmitted. By referring to these values, thetransmission error rate can be extracted with respect to eachdestination. Based on the destination-by-destination transmission errorrates, the Rate Strategy of RTS information can be determined. Morespecific description will be given. The lower threshold value and theupper threshold value of transmission error rate are defined in advance.If the transmission error rate exceeds the upper threshold value, thevalue for Rate Strategy is decremented in the passive direction andheld. If the transmission error rate falls below the lower thresholdvalue, the value for Rate Strategy is incremented in the aggressivedirection and held. When an RTS is transmitted, this held Rate Strategyis transcribed into the Rate Strategy field in RTS. If the held valuefor Rate Strategy is changed, it is important to reset the count valuesfor obtaining the transmission error rate.

According to the above-mentioned procedure, the transmission rate isdetermined from the receive signal quality of RTS. Thus, control can becarried out by dual loop wherein the applied transmission rate is biasedin accordance with the error rate (bit error rate (BER) or packet errorrate (PER)) monitored by the transmitting end.

In addition, as a technique different from the foregoing, the followingconstitution is possible: the transmission error rate is measured by thereceiving end, not by the data (RTS) transmitting end. In this case, theabove-mentioned monitoring of error rate is carried out at the receivingend, and similarly dual loop control is carried out wherein the appliedtransmission rate is biased in accordance with the error rate. If theRTS/CTS procedure is not used together in the technique, however,certain risk is involved. This is because, in an environment whereerrors frequently occur, the receiving end cannot even recognize thatpackets are transmitted. Where RTS/CTS is used together, the error rateis computed with the packets received after CTS is transmitted taken asthe population parameter.

The quintessence of Rate Strategy is to provide a methodology forcommunicating auxiliary information for selecting and determining a rateon the grounds of the transmitting end. Where the receiving end measuresthe transmission error rate, mentioned above, or in the like cases, thetransmitting end may set the value of Rate Strategy due to other factorsthan error rate. For example, the transmitting end may set the value ofRate Strategy due to the difference in transmission power between RTSpacket and data packet. Or, it may set the value of Rate Strategy due tothe causal relationship between transmission rate and transmissionpower. (Refer to Japanese Patent Application No. 2003-6973specification, already assigned to the present applicant, for example.)

D. Basic CSMA Procedure

In a wireless communication system according to the present invention,access control based on CSMA is carried out with the RTS/CTS procedureused together. FIG. 7 illustrates an example of the operation sequenceof the RTS/CTS procedure in the present invention. In this figure, thefollowing communication environment is assumed: four communicationstations, STA2, STA0, STA1, and STA3, exist, and communication stationadjoining to each other in the figure are located within the range ofradio waves. Here, it is assumed that STA0 desires to transmitinformation to STA1.

STA0 confirms that the medium is clear for a certain period (from timeT0 to time T1) according to, for example, the ordinary CSMA procedure.Then, STA0 starts the transmission of a packet containing RTSinformation to STA1 at time T1.

In the Duration field in the packet containing the RTS information, thetime that lapses before the reception of a packet containing CTSinformation, transmitted as the outcome of the reception of the RTSinformation, is completed, is described. (That is, the time from T2 toT4 is described.) At this time, as the length of time described in theDuration field, the time that lapses before time T4 may be given somemargin and the time until time T5 or so may be described. Or, the timeuntil time slightly before time T4 may be described.

STA0 holds a list of information on the length of data unit it tries totransmit to STA1. This list is designated as “transmit data unitcandidate list.”

In case the RTS SMH adopts the first example of constitution mentionedabove, in the Length field in the RTS SMH, the lengths (e.g. number ofbits and number of bytes) of data units in the transmit data unitcandidate list are described as data length information by N pieces.

In case the RTS SMH adopts the second example of constitution, in theLength field in the RTS SMH, the sum of the lengths of one or more dataunits in the transmit data unit candidate list is described as datalength information.

In case the RTS SMH adopts the third example of constitution, in theLength field in the RTS SMH, the length of time required fortransmitting the data units in the transmit data unit candidate list isdescribed as data length information. The length of time fluctuatesdepending on the data rate at which transmission is carried out.Therefore, if transmission is carried out at several data rates, aplurality of lengths of time are described sometimes.

In case the RTS SMH adopts the fourth example of constitution, in theNumber Of Data Unit field (defined in place of the Length field) in RTSSMH, the number of data units in the transmit data unit candidate listis described as data length information.

This packet containing RTS information is received also by STA2 which ispositioned in the vicinity of STA0. STA2 checks the destination of thecommon MAC header based on the transmission rate information in the PSDUportion obtained from the RATE field in the PLCP header. If STA2 cannotconfirm that the packet is directed to itself, it recognizes time T2from the RATE field and the LENGTH field in the PLCP header. Further,STA2 refers to the Duration field in the common MAC header, and thusrecognizes time T4. Then, STA2 sets an NAV and brings itself intransmission prohibited state until time T4.

This packet containing RTS information is received also by STA1 which isthe intended destination. STA1 checks the destination of the common MACheader based on the transmission rate information in the PSDU portionobtained from the RATE field in the PLCP header, and recognizes that thepacket is directed to itself. Further, when STA1 successfully decodesthe sub MAC header, it recognizes that there is a request from STA0 tosend data packets, directed to itself. Then, STA1 determines atransmission rate based on information obtained from the received packetcontaining RTS information.

Further, STA1 refers to the Length field (or the Number Of Data Unitfield) in RTS SMH, and thereby obtains the data length information onthe data to be received.

In case the RTS SMH adopts the above-mentioned first example ofconstitution, STA1 obtains a list of information on the lengths of dataunits to be received. This list is designated as “receive data unitcandidate list.” STA1 refers to this list in sequence. Thereby, itcounts the number of data units which can be received if reception iscarried out at the determined transmission rate and a reception periodto the extent that the period indicated in the Max Duration field in RTSSMH is not exceeded. (This reception period is the time period untiltime T6 when the transmission of data packets is completed.)

In case the RTS SMH adopts the second example of constitution, STA1obtains the total length of one or more data units received. STA1 countsthe reception period to the extent that the period indicated in the MaxDuration field in RTS SMH is not exceeded. At this time, considerationis given to whether data unit groups of the indicated length can bereceived if reception is carried out at the determined transmissionrate.

In case the RTS SMH adopts the third example of constitution, STA1obtains time length candidates of data packets received. STA1 extractsone reception period (time length) corresponding to the determinedtransmission rate from the Length field.

In case the RTS SMH adopts the fourth example of constitution, STA1obtains information on the number of data units received from the NumberOf Data Unit field. Where the length of data units is uniquely fixed,this information is equivalent to the “receive data unit candidatelist.” STA1 refers to the length and number of data units. Thereby, itcounts the number of data units which can be received if reception iscarried out at the determined transmission rate and the reception periodto the extent that the period indicated in the Max Duration field in RTSSMH is not exceeded.

If STA1 desires to limit the reception period on the grounds of itselfat this time, it sets this reception period as the threshold value ofreception period, together with the period indicated by Max Duration,sometimes. Further, STA1 transmits a packet containing CTS information.Thus, it transcribes the value obtained by the reception period computedhere into the Duration field in that packet, and transcribes thetransmission rate class determined as mentioned above into the RATEfield.

The packet itself containing CTS information is transmitted at atransmission rate most superior in noise tolerance in many cases. Thus,the possibility that a packet containing CTS information cannot bedecoded is reduced as much as possible.

If STA1 fails to decode a packet containing RTS information, a packetcontaining CTS information is not returned by due time T4. Therefore,STA0 retries to transmit the packet containing RTS information inaccordance with the random backoff procedure. At this time, STA2 putsdown the NAV at time T4, and transitions into normal state in which itcan transmit. Thus, damage resulting from that the packet containing CTSinformation is not returned is minimized.

If STA1 succeeds in decoding the packet containing RTS information, apacket containing CTS information is returned to STA0 at time T4 asscheduled in accordance with the above-mentioned procedure.

This packet containing CTS information is received also by STA3 which ispositioned in the vicinity of STA1. STA3 decodes the Duration field andthe like in the packet containing CTS information, and refers to the RAaddress. If STA3 cannot confirm that the packet is directed to itself asthe result, it sets an NAV and stops transmission over the timeindicated in the Duration field or the like. As a result, STA3 is keptin transmission prohibited state until time T6 which is the timeindicated in the Duration field.

This packet containing CTS information is received also by STA0 which isthe intended destination. STA0 decodes the PSDU, and thereby recognizesthe following: STA1 is ready to receive and it desires to receive at thetransmission rate indicated in the RATE field during the periodindicated by Duration. STA0 calls data units which can be transmitted atthe specified transmission rate during the specified period, beginningwith the first one in the transmit data unit candidate list in sequence.Then, it generates a transmit data packet, and transmits it.

The data packet transmission is completed at time T6, and STA3 which isa neighbor station of STA1 refrains from transmission until T6.Therefore, the reception by STA1 is not hindered.

E. Applications of CSMA Procedure

FIG. 8 to FIG. 10 and FIG. 21 to FIG. 23 illustrate a number of examplesof the applications of transmitting and receiving procedures accordingto the present invention. Here, examples of the applications of thecommunication method according to the present invention will bedescribed in detail based on the CSMA procedure described referring toFIG. 7.

E-1. First Example of Application

FIG. 8 illustrates the first example of application of transmitting andreceiving procedures in a wireless communication system according to thepresent invention. The figure illustrates an example in which two dataunits are transmitted from STA0 to STA1. In this example, the number ofdata units which can be transmitted in one packet is limited to one, anddelay ACK (Del-ACK) or immediate ACK (Im-ACK) is specified as the casemay be.

STA0 starts the procedure for transmitting data units at time T0, andstarts backoff count in accordance with the CSMA procedure for acquiringtransmission right. The backoff count is completed at T1, and it isconfirmed that the medium is clear during this period. Therefore, STA0transmits a packet containing RTS information.

After STA1 receives the RTS directed to itself, it returns a packetcontaining CTS information at time T2 based on the information describedin the RTS SMH.

After STA0 receives the CTS directed to itself, it transmits a datapacket at time T3. At this time, STA0 describes in the ACK Type portionof the data packet information indicating it desires that ACK should bereturned by delay ACK (Del-ACK). At the same time, STA0 describes in theMore Bit portion information indicating that more data unit is stored.This data packet is received by STA1 without error.

To further transmit the stored data unit directed to STA1, STA0 waitsfor a random time, and then transmits a packet containing RTSinformation at time T5. At this point of time, STA0 has not received theACK of the data unit it previously transmitted. Therefore, STA0generates a transmit data unit candidate list so that the previouslytransmitted data unit is included, and constitutes an RTS SMH based onthe list.

After STA1 receives it, its returns a packet containing CTS informationat time T6 based on the information described in the RTS SMH. At thistime, STA1 refers to the RTS Sequence and RTS Received MAP in the RTSSMH, and thereby recognizes that STA0 has not recognized the receptionconfirmation of the previously received data unit. Then, STA1 decides toinclude ACK information in the packet containing CTS information. Or,STA1 determines to include ACK information because of it's holdinguntransmitted ACK directed to STA0. Further, STA1 deletes the alreadyreceived packet from the receive data unit candidate list to update thelist, and then generates CTS information.

As a result, STA1 returns a packet (i.e. CTS with ACK multiplexed)containing the CTS information and the ACK information at time T6. ForACK information, the ACK information corresponding to the data flowspecified by the received RTS information is described.

After receiving the packet containing the CTS information and ACKinformation, STA0 operates as follows: based on the ACK information,STA0 deletes the data unit already received by STA1 from the transmitdata unit candidate list it holds; STA0 generates anew transmit dataunit candidate list, and determines a transmit data unit based on thenew list. STA0 transmits the thus generated data packet at T7. At thistime, it describes in the ACK Type portion of the data packetinformation indicating it desires that ACK should be returned byimmediate ACK. At the same time, it describes in the More Bit portioninformation indicating that a data unit to be transmitted is not presentany more.

When STA1 receives the data packet, it recognizes that it is desired toimmediately transmit an ACK, from the ACK Type portion in the DATA SMH.Then, STA1 returns an immediate ACK (Im-ACK) packet corresponding to therelevant data flow at time T8.

In this example of application, the Duration field in each packet is setso that it indicates the time period until the time indicated by arcuatearrow in FIG. 8.

E-2. Second Example of Application

FIG. 21 illustrates the second example of application of transmittingand receiving procedures in a wireless communication system according tothe present invention. The figure illustrates an example in which twodata units are transmitted from STA0 to STA1. This example is on theassumption that the number of data units which can be transmitted in onepacket is limited to one and immediate ACK (Im-ACK) is constantlyapplied.

STA0 starts the procedure for transmitting data units at time T0, andstarts backoff count in accordance with the CSMA procedure for acquiringtransmission right. The backoff count is completed at T1, and it isconfirmed that the medium is clear during this period. Therefore, STA0transmits a packet containing RTS information.

After STA1 receives the RTS directed to itself, it returns a packetcontaining CTS information at time T2 based on the information describedin the RTS SMH.

After STA0 receives the CTS directed to itself at time T2, it transmitsa data packet at time T3. At this time, STA0 describes in the ACK Typeportion of the data packet information indicating it desires that ACKshould be returned by immediate ACK (Im-ACK).

However, STA1 detects an error when it receives the data packet andcannot accurately extract the data. Though STA1 should return an ACK,therefore, it does not transmit an ACK at time T4. (The ACK in thefigure is not transmitted.)

Since STA0 cannot receive ACK at due time, it judges that some error hasoccurred in the previous data. Thus, to retransmit the still stored dataunit directed to STA1, STA0 waits for a random time, and then transmitsa packet containing the RTS information at time T5. As mentioned above,STA0 has not received the ACK of the data it previously transmitted. Atthis time, therefore, STA0 describes in the ACK Request portion in theRTS SMH that it requests that ACK information should be added. (That is,STA0 transmits an RTS with an ACK request multiplexed.)

After receiving this, STA1 returns a packet containing CTS informationat time T6 based on the information described in the RTS SMH. At thistime, STA1 refers to the ACK Request portion in the RTS SMH, and therebyrecognizes that STA0 is requesting that ACK information should betransmitted. Then, STA1 decides to include ACK information in the packetcontaining CTS information. Further, STA1 deletes the already receivedpacket from the receive data unit candidate list to update the list, andthen generates CTS information.

As a result, STA1 returns a packet (i.e. CTS with ACK multiplexed)containing the CTS information and the ACK information at time T6. ForACK information, the ACK information corresponding to the data flowspecified by the received RTS information is described.

After receiving the packet containing the CTS information and the ACKinformation, STA0 operates as follows: based on the ACK information,STA0 deletes the data unit already received by STA1 from the transmitdata unit candidate list it holds; STA0 generates a new transmit dataunit candidate list, and determines a transmit data unit based on thenew list. (In this example, STA0 retransmits the data unit ittransmitted at time T3.) STA0 transmits the thus generated data packetat time T7. At this time, it describes in the ACK Type portion of thedata packet information indicating it desires that ACK should bereturned by immediate ACK.

When STA1 receives the data packet, it recognizes that it is desired toimmediately transmit an ACK, from the ACK Type portion in the DATA SMH.Then, STA1 returns an immediate ACK (Im-ACK) packet corresponding to therelevant data flow at time T8.

In this example of application, the Duration field in each packet is setso that it indicates the time period until the time indicated by arcuatearrow in FIG. 8.

E-3. Third Example of Application

FIG. 22 illustrates the third example of application of transmitting andreceiving procedures in a wireless communication system according to thepresent invention. The figure illustrates an example in which one dataunit is transmitted from STA0 to STA1. This example is on the assumptionthat the number of data units which can be transmitted in one packet islimited to one and immediate ACK (Im-ACK) is constantly applied.

STA0 starts the procedure for transmitting data units at time T0, andstarts backoff count in accordance with the CSMA procedure for acquiringtransmission right. The backoff count is completed at T1, and it isconfirmed that the medium is clear during this period. Therefore, STA0transmits a packet containing RTS information.

After STA1 receives the RTS directed to itself, it returns a packetcontaining CTS information at time T2 based on the information describedin the RTS SMH.

After STA0 receives the CTS directed to itself at time T2, it transmitsa data packet at time T3. At this time, STA0 describes in the ACK Typeportion of the data packet information indicating it desires that ACKshould be returned by immediate ACK (Im-ACK). At the same time, STA0describes in the More Bit portion information indicating that a transmitdata unit is not stored any more. This data packet is received by STA1without error.

STA1 returns the ACK of the received data. However, STA0 detects anerror when it receives the ACK and cannot accurately extract the data.Since STA0 cannot receive ACK at due time, it judges that some error hasoccurred in the previous data. Thus, to retransmit the still stored dataunit directed to STA1, STA0 waits for a random time, and then transmitsa packet containing RTS information for data retransmission at T5. Asmentioned above, STA0 has not received the ACK of the data it previouslytransmitted. At this time, therefore, STA0 describes in the ACK Requestportion in the RTS SMH that it requests that ACK information should beadded. That is, STA0 transmits an RTS with an ACK request multiplexed.

After receiving this, STA1 returns a packet containing CTS informationat time T6 based on the information described in the RTS SMH. At thistime, STA1 ref ers to the ACK Request portion in the RTS SMH, andthereby recognizes that STA0 is requesting that ACK information shouldbe transmitted. Then, STA1 decides to include ACK information in thepacket containing CTS information. Further, STA1 recognizes that, as theresult of the already received packet being deleted from the receivedata unit candidate list, all the data STA0 desires to transmit has beenalready received. Thus, STA1 returns only an ACK, and does not returnCTS information.

As a result, STA1 returns a packet containing only ACK information attime T6. For ACK information, the ACK information corresponding to thedata flow specified by the received RTS information is described.

After receiving the packet containing the ACK information, STA0 judgesthat all the transmit data unit directed to STA1 it has held has beenalready transmitted, and terminates the transaction.

In this example of application, the Duration field in each packet is setso that it indicates the time period until the time indicated by arcuatearrow in FIG. 8.

E-4. Fourth Example of Application

FIG. 23 illustrates the fourth example of application of transmittingand receiving procedures in a wireless communication system according tothe present invention. The figure illustrates an example in which twodata units are transmitted from STA0 to STA1. This example is on theassumption that the number of data units which can be transmitted in onepacket is limited to one and it is permitted to continuously transmit aplurality of data packets.

STA0 starts the procedure for transmitting data units at time T0, andstarts backoff count in accordance with the CSMA procedure for acquiringtransmission right. The backoff count is completed at T1, and it isconfirmed that the medium is clear during this period. Therefore, STA0transmits a packet containing RTS information.

After STA1 receives the RTS directed to itself, it returns a packetcontaining CTS information at time T2 based on the information describedin the RTS SMH.

After STA0 receives the CTS directed to itself, it transmits a datapacket at time T3. At this time, STA0 describes in the ACK Type portionof the data packet information indicating it desires that ACK should bereturned by delay ACK (Del-ACK). At the same time, STA0 describes in theMore Bit portion information indicating that more transmit data unit isstored. This data packet is received by STA1 without error.

Further, STA0 judges whether STA0 can complete transmission within theperiod indicated in the Duration field in the CTS, transmitted by STA1at time T2, if STA0 continuously transmits data packets, starting attime T4. In the example illustrated in the figure, STA0 judges that, ifit transmits the next data packet, it can complete data transmissionwithin the period indicated in the Duration field in the CTS. Then, STA0further transmits a stored data unit directed to STA1 at time T4. Atthis time, STA0 describes in the ACK Type portion of the data packetinformation indicating it desires that ACK should be returned byimmediate ACK (Im-ACK). At the same time, STA0 describes in the More Bitportion information indicating a transmit data unit is not stored anymore. This data packet is received by STA1 without error.

When STA1 receives the data packet, it recognizes that it is desired toimmediately transmit an ACK, from the ACK Type portion in the DATA SMH.Then, STA1 returns an immediate ACK (Im-ACK) packet corresponding to therelevant data flow at time T5.

In this example of application, the Duration field in each packet is setso that it indicates the time period until the time indicated by arcuatearrow in FIG. 8.

E-5. Fifth Example of Application

FIG. 9 illustrates the fifth example of application of transmitting andreceiving procedures in a wireless communication system according to thepresent invention. The figure also illustrates an example in which twodata units are transmitted from STA0 to STA1. In this example, thenumber of data units which can be transmitted in one packet is limitedto one. With respect to both the data units, immediate ACK is specified,and the transmitter station is permitted to continuously transmitpackets.

STA0 starts the procedure for transmitting data units at time T0, andstarts backoff count in accordance with the CSMA procedure for acquiringtransmission right. The backoff count is completed at T1, and it isconfirmed that the medium is clear during this period. Therefore, STA0transmits a packet containing RTS information.

After STA1 receives the RTS directed to itself, it returns a packetcontaining CTS information at time T2 based on the information describedin the RTS SMH.

After STA0 receives the CTS directed to itself, it transmits a datapacket at time T3. At this time, STA0 describes in the ACK Type portionof the data packet information indicating it desires that ACK should bereturned by immediate ACK. At the same time, STA0 describes in the MoreBit portion information that more transmit data unit is stored. Further,STA0 adds RTS information for transmitting another data unit. That is,STA0 transmits a data packet with the RTS multiplexed. This data packetis received by STA1 without error.

STA1 recognizes that the data packet contains RTS information, and thentries to transmit a packet containing CTS information. At this time,STA1 refers to RTS Sequence and RTS Received MAP, and thereby recognizesthat STA0 has not recognized the acknowledgement of the data unitpreviously received. Then, STA1 decides to include ACK information inthe packet containing CTS information. Further, STA1 generates a receivedata unit candidate list based on the information described in the RTSSMH. At this time, STA1 deletes the already received packet from thereceive data unit candidate list to update the list, and then generatesCTS information. Further, as ACK information, STA1 describes the ACKinformation corresponding to the data flow specified by the received RTSinformation. Thus, STA1 returns a packet containing ACK information andCTS information (i.e. a CTS packet with an ACK multiplexed) at time T4.

After STA0 receives the packet containing ACK information and CTSinformation, it tries to transmit a data unit based on the CTSinformation. Based on the ACK information, STA0 deletes the data unitalready received by STA1 from the transmit data unit candidate list itholds. Then, STA0 generates a new transmit data unit candidate list, anddetermines a transmit data unit based on this new list. At this time,STA0 describes in the ACK Type portion of the data packet informationindicating it desires that ACK should be returned by immediate ACK. Atthe same time, STA0 describes in the More Bit portion informationindicating that a data unit to be transmitted is not present any more.STA0 transmits the thus generated data packet at time T5.

When STA1 receives the data packet with ACK information multiplexed, itrecognizes that it is desired to immediately transmit an ACK, andreturns a corresponding ACK information packet at time T6.

In this example of application, the Duration field in each packet is setso that it indicates the time period until the time indicated by arcuatearrow in FIG. 9. As the Duration value of the packet transmitted at timeT6, zero indicating NULL is placed.

E-6. Sixth Example of Application

FIG. 10 illustrates the sixth example of application of transmitting andreceiving procedures in a wireless communication system according to thepresent invention. This figure illustrates an example of two-waycommunication wherein two data units are transmitted from STA0 to STA1and simultaneously two data units are transmitted from STA1 to STA0. Inthis example, the number of data units which can be transmitted in onepacket is limited to one, and it is permitted to multiplex RTS into CTS.

STA0 starts the procedure for transmitting data units at time T0, andstarts backoff count in accordance with the CSMA procedure for acquiringtransmission right. The backoff count is completed at T1, and it isconfirmed that the medium is clear during this period. Therefore, STA0transmits a packet containing RTS information. In the RTS/CTS Muxportion in the RTS SMH, information is set which indicates that RTS maybe multiplexed into CTS.

After STA1 receives the RTS information, it returns a packet containingCTS information at time T2 based on the information described in the RTSSMH. At this time, STA1 recognizes from the RTS/CTS Mux portion in theRTS SMH that RTS may be multiplexed into CTS, and it holds data unitsdirected to STA0. Therefore, STA1 generates RTS information fortransmitting these data units to STA0. As a result, a packet containingCTS information and RTS information (i.e. a CTS packet with an RTSmultiplexed) is transmitted.

After STA0 receives the packet containing CTS information and RTSinformation, it tries to transmit a data unit at time T3 based on theCTS information. At this time, STA0 describes in the ACK Type portion ofthe data packet information indicating it desires that ACK should bereturned by immediate ACK. At the same time, STA0 describes in the MoreBit portion information indicating that more transmit data unit isstored. Further, STA0 adds RTS information for transmitting another dataunit. Since RTS information is also contained in the receive packet,STA0 generates CTS information in response thereto, and adds the CTSinformation.

As a result, a packet in which data, the RTS information, and the CTSinformation are multiplexed is transmitted from STA0 at time T3. (Thatis, a data packet in which CTS in response to RTS from STA1 and RTS fortransmitting the subsequent data unit are multiplexed is transmitted.)This data packet is received by STA1 without error.

STA1 receives the packet with the data, RTS information, and CTSinformation multiplexed, and recognizes that this packet contains RTSinformation. Then, STA1 tries to transmit a packet containing CTSinformation. At this time, STA1 refers to RTS Sequence and RTS ReceivedMAP, and thereby recognizes that STA0 has not recognized theacknowledgement of the data unit previously received. Thus, STA1 decidesto include ACK information in the packet containing CTS information.Further, STA1 generates a receive data unit candidate list based on theinformation described in the RTS SMH. At this time, STA1 deletes thealready received packet from the receive data unit candidate list toupdate the list, and then generates CTS information. Further, as ACKinformation, STA1 describes the ACK information corresponding to thedata flow specified by the received RTS information. Further, STA1recognizes that the packet contains CTS information, and transmits adata unit as well based on the CTS information. STA1 describes in theACK Type portion in the DATA SMH information indicating it desires thatACK should be returned by immediate ACK. At the same time, STA1describes in the More Bit portion information indicating that moretransmit data unit is stored. Further, STA1 adds RTS information fortransmitting another data unit.

Thus, STA1 returns a packet containing ACK information, CTS information,a data unit, and RTS information at time T4. That is, STA1 multiplexesand transmits the following: a data packet in response to the receptionof CTS from STA0; a CTS in response to an RTS from STA0; an RTS fortransmitting the subsequent data unit; and an ACK corresponding to thereception of a data unit from STA0.

STA0 receives the packet containing ACK information, CTS information, adata unit, and RTS information, and tries to transmit a data unit basedon the CTS information. Based on the ACK information in the receivedpacket, STA0 deletes the data unit already received by STA1 from thetransmit data unit candidate list it holds. Then, STA0 generates a newtransmit data unit candidate list, and determines a transmit data unitbased on this new list. At this time, STA0 describes in the ACK Typeportion of the data packet information indicating it desires that ACKshould be returned by immediate ACK. At the same time, STA0 describes inthe More Bit portion information indicating that a data unit to betransmitted is not present any more. Further, since STA0 extracted thenew data unit from the received packet without error, it decides toreturn ACK information. Further, since the received packet contains RTSinformation as well, STA0 generates CTS information correspondingthereto, and also decides to transmit the CTS information. Thegenerating procedure for CTS information is the same as described above;therefore, its description will be omitted here.

Thus, STA0 returns the packet containing a data unit, ACK information,and CTS information at time T5. That is, STA0 multiplexes and transmitsthe following: a data unit in response to a CTS from STA1; a CTS inresponse to an RTS from STA1; and an ACK corresponding to a data unitreceived form STA1.

When STA1 receives the packet containing a data unit, ACK information,and CTS information, it extracts the data unit, and decides to transmitACK information corresponding thereto. Further, when STA1 recognizesthat the packet contains CTS information and ACK information, it updatesthe transmit data unit candidate list based on the ACK information.Then, STA1 decides to transmit a data unit as well based on the CTSinformation. Thus, STA1 returns a packet containing ACK information anda data unit (i.e. a data packet with the ACK of the data unit receivedfrom STA0 multiplexed) at time T6.

After STA0 receives the packet containing ACK information and a dataunit, it refers to the ACK Type portion in the DATA SMH, and therebyrecognizes that it is desired to immediately transmit an ACK. Then, STA0returns a corresponding ACK information packet at time T7.

In the above example, the Duration field in each packet is set so thatit indicates the time period until the time indicated by arcuate arrowin FIG. 10. Since the data length is determined beforehand when CTSinformation is generated, each Duration value can be set.

Whether RTS information or ACK information is multiplexed in a returnedpacket is unknown; therefore, Duration cannot be exactly specifiedsometimes. In preparation for such a case, a slightly larger valueallowing a margin is set for the Duration value sometimes.

Further, there are cases where a Duration value is set on the assumptionthat RTS information or ACK information is not multiplexed and an actualpacket is transmitted over a period slightly longer than the Durationvalue. Since the length of this information is not so large, however,great loss will not result. As the Duration value of the packettransmitted at time T8, zero indicating NULL is placed.

In the above-mentioned embodiments E-1 to E-6, it is judged whether toset “information indicating RTS may be multiplexed into CTS” in theRTS/CTS Mux portion in the RTS SMH. This judgment can also be made basedon the history of communication between RTS transmitting station andstation as its destination, or the packets received from the station asthe destination of the relevant RTS in the past. Referring to therelevant drawings, description will be given below to some examples inwhich control is carried out in accordance with whether the station asthe destination of RTS holds data it desires to transmit to the RTStransmitting station or not.

In the example illustrated in FIG. 24, STA1 transmits RTS (P0), STA0returns CTS (P1), STA1 transmits DATA (P2), and STA0 returns ACK (P3).

Here, it is assumed that, in the More Bit portion in the DATA SMH of thepacket P2 containing DATA, it is described that STA1 still holds datadirected to STA0.

STA0 holds the information in the More Bit portion of data most recentlyreceived from STA1, and can thereby recognize that STA1 holds data STA1desires to transmit to STA0.

If STA0 recognizes that STA1 holds data directed to STA0 when STA0transmits an RTS to STA1, it sets in the RTS/CTS Mux portion in the RTSSMH “information indicating that RTS may be multiplexed into CTS.”

FIG. 24 illustrates a case where this RTS is transmitted as P4. Inresponse thereto, STA1 returns a packet containing CTS and RTS SMHrelated to its own data transmission, that is, a packet (P5) wherein aCTS is multiplexed into an RTS. Then, STA0 transmits a packet containingdata in response to the received CTS SMH and a CTS SMH corresponding tothe received RTS SMH, that is, a packet (P6) where in data and CTS aremultiplexed. Subsequently, STA1 transmits a packet containing ACK SMHcorresponding to the received data and data corresponding to thereceived CTS SMH, that is, a packet (P7) wherein data and ACK aremultiplexed. Further, STA0 transmits a packet (P8) containing ACK SMHcorresponding to the received data.

In the example illustrated in FIG. 25, as in the example illustrated inFIG. 24, STA1 transmits RTS (P0), STA0 returns CTS (P1), STA1 transmitsDATA (P2), and STA0 returns ACK (P3).

Here, it is assumed that, in the More Bit portion in the DATA SMH of thepacket P2 containing DATA, it is described that STA1 still holds datadirected to STA0.

STA0 holds the information in the More Bit portion of data most recentlyreceived from STA1, and can thereby recognize that STA1 holds data STA1desires to transmit to STA0.

In this case, the following can occur even if data directed to STA1 isnot present in STA0: because STA1 holds data STA1 desires to transmit toSTA0, STA0 transmits an RTS wherein “information indicating that RTS maybe multiplexed into CTS” is set in the RTS/CTS Mux portion in the RTSSMH. At this time, STA0 writes in the other fields in the RTS SMHinformation indicating that STA0 has no data to transmit.

This RTS is a dummy RTS which is different from the RTS proper whichrequests data transmission of the destination of data transmission.Other surrounding stations than STA1 receive this dummy RTS, and therebyset an NAV and refrain from transmission only for the Duration describedin the RTS. Since the other surrounding stations refrain fromtransmission, it is easier for STA1 to transmit RTS for transmitting thesubsequent data unit. That is, STA0 transmits a dummy RTS, and canthereby make it easier for STA1 to acquire RTS transmission right. As aresult, STA0 can efficiently receive the subsequent data unit from STA1.

FIG. 25 illustrates a case where this dummy RTS is transmitted as P4. Inresponse thereto, STA1 performs the operation of transmitting thesubsequent data unit in accordance with the ordinary RTS/CTS procedure.Specifically, STA1 returns a packet (P5) containing RTS SMH related toits own data transmission, and STA0 transmits a packet (P6) containingCTS SMH corresponding to the received RTS SMH. STA1 transmits a packet(P7) containing data corresponding to the received CTS SMH, and STA0transmits a packet (P8) containing ACK SMH corresponding to the receiveddata.

In the example illustrated in FIG. 26, STA1 transmits RTS (P0) to STA0,and STA0 can receive it without error. However, a problem arises. Duringthis period of time, STA0 interpreted the Duration field in a packettransmitted from another station (hidden terminal (not shown) for STA1),and thereby has set an NAV. Therefore, STA0 cannot return a CTS. STA0receives the RTS packet (P0), and thereby recognizes that STA1 holdsdata STA1 desires to transmit STA0.

In the example illustrated in FIG. 25, STA1 sets More bit, and therebySTA0 recognizes that STA1 has the subsequent data unit. The exampleillustrated in FIG. 26 is different in that:

STA0 receives an RTS from STA1 and thereby recognizes that STA1 has thesubsequent data unit. In the latter, STA0 sets an NAV and thus cannotreturn a CTS. Therefore, though STA1 has the subsequent data unit, itsdata transmission operation is stopped.

In this case, the following can occur even if data directed to STA1 isnot present in STA0: because STA1 holds data STA1 desires to transmit toSTA0, STA0 transmits an RTS wherein “information indicating that RTS maybe multiplexed into CTS” is set in the RTS/CTS Mux portion in the RTSSMH. At this time, STA0 writes in the other fields in the RTS SMHinformation indicating that STA0 has no data to transmit. (If datadirected to STA1 is present in STA0, the same operation takes placethough this is not shown in the figure. That is, STA0 transmits an RTSwherein “information indicating that RTS may be multiplexed into CTS” isset in the RTS/CTS Mux portion in the RTS SMH.)

This RTS is a dummy RTS which is different from the RTS proper. Whenother surrounding stations than STA1 receive this dummy RTS, they set anNAV and refrain from transmission. Since the other surrounding stationrefrain from transmission, it is easier for STA1 to transmit RTS fortransmitting the subsequent data unit. That is, STA0 transmits a dummyRTS, and can thereby make it easier for STA1 to acquire RTS transmissionright. As a result, STA0 can efficiently receive the subsequent dataunit from STA1 (same as above).

FIG. 26 illustrates a case where this RTS is transmitted as P4. Inresponse thereto, STA1 performs the operation of transmitting thesubsequent data unit in accordance with the ordinary RTS/CTS procedure.Specifically, STA1 returns a packet (P5) containing RTS SMH related toits own data transmission, and STA0 transmits a packet (P6) containingCTS SMH corresponding to the received RTS SMH. STA1 transmits a packet(P7) containing data corresponding to the received CTS SMH, and STA0transmits a packet (P8) containing ACK SMH corresponding to the receiveddata.

As an example of utilization different from the foregoing, “informationindicating that RTS may be multiplexed into CTS” may be always set inthe RTS/CTS Mux portion in the RTS SMH.

F. Procedure for Acknowledgement of Sequence Number

FIG. 11 illustrates the acknowledgement procedure related to data otherthan data flow 0 assumed in the present invention. The figure shows aconcrete example of the interaction of individual fields which occurswhen selective acknowledgement is carried out. The example is on theassumption that the Received MAP field is constituted of four bits forthe purpose of simplifying the explanation; however, the presentinvention is not limited to this constitution.

The data transmitting end transmits data units of sequence numbers “0,”“1,” “2,” and “3” from the transmit data unit candidate list. It isassumed that the data unit #2 is received with an error therein.

In the ACK Sequence field in the acknowledgement ACK returned by thedata receiving end, “1,” which indicates that the data units of up to #1could be completely received, is placed. With the data unit #2 which isthe data unit subsequent to #1 taken as the basis (as MSB), “0100” isplaced in the Received MAP field. “0100” indicates the following: thereception of the data unit #2 failed (0); that of the data unit #3succeeded (1); that of the data unit #4 failed (0); and that of the dataunit #5 also failed (0).

When the data transmitting end receives this, it recognizes that thedata units of up to #1 were received and the data unit #3 was alsoreceived. In the next data transmission, the data transmitting enddeletes them from the transmit data unit candidate list, and transmitsthe data units of sequence numbers “2,” “4,” “5,” and “6.” It is assumedthat the data units #4 and #5 are received with an error therein.

In the ACK Sequence field in the acknowledgement ACK returned by thedata receiving end, “3,” which indicates that the data units of up to #3could be completely received, is placed. With the data unit #4 which isthe data unit subsequent to #3 taken as the basis (as MSB), “0010” isplaced in the Received MAP field. “0010” indicates the following: thereception of the data unit #4 failed (0); that of the data unit #5 alsofailed (0); that of the data unit #6 succeeded (1); and that of the dataunit #7 failed (0).

Thus, the ACK Sequence field and the Received MAP are used together, andthereby ARQ (Automatic Repeat reQuest) is carried out. If the datatransmitting end does not support selective acknowledgement, thefollowing procedure is taken: when it receives an ACK and updates thetransmit data unit candidate list, it disregards Received MAP. If thedata receiving end does not support selective acknowledgement, thefollowing procedure is taken: it does not try to decode the data unitwith an error therein and the subsequent pieces of data for linkage withthe ACK Sequence field, and places zero in all the bits of the ReceivedMAP field. Thus, even if communication is carried out between acommunication station which supports selective acknowledgement and acommunication station which does not, the communication can bemaintained without failure.

G. Procedure for Acknowledgement of Sequence Number with RTS/CTSProcedure Used Together

FIG. 12 illustrates another concrete example of the interaction ofindividual fields which occurs when selective acknowledgement is carriedout with RTS/CTS used together. Here, to give description with attentionfocused only on the portions related to the sequence number and theReceived MAP, other fields will not be mentioned. However, in reality,procedures using other fields, which are described elsewhere, are alsocarried out. The example is on the assumption that the Received MAPfield is constituted of four bits for the purpose of simplifying theexplanation; however, the present invention is not limited to thisconstitution. With respect to RTS SMH, this example is on the assumptionthat the first or fourth example of constitution is adopted.

The data transmitting end transmits an RTS which indicates that the dataunits of sequence numbers “3,” “4,” “5” “6” . . . , are entered in thetransmit data unit candidate list. At this time, in the RTS Sequencefield in the RTS SMH, “3” is placed which indicates the leading dataunit, and in the RTS Received MAP field, “0000” is placed whichindicates that all the data units #4 to #7 are transmit candidates.

Since all the data units indicated by the RTS have not received yet, thedata receiving end does not return an ACK but returns a CTS and waitsfor incoming data units.

It is assumed that the data units #4 and #5 are received with an errortherein. Further, it is assumed that delay ACK is specified for thesedata. Because of delay ACK, the receiving end does not immediatelyreturn an ACK.

To further transmit data, the data transmitting end transmits an RTSagain which indicates that the data units #3, #4, #5, #6, . . . areentered in the transmit data unit candidate list. Similarly with theforegoing, in the RTS Sequence field in the RTS SMH, “3” is placed whichindicates the leading data unit, and in the RTS Received MAP, “0000” isplaced which indicates that all the data units #4 to #7 are transmitcandidates.

Since some of the data units indicated by the RTS have been alreadyreceived, the data receiving end adds ACK information when it returns aCTS. At this time, “3” is placed in the ACK SEQUENCE field in the ACKSMH, indicating that the data units of up to #3 could be completelyreceived. With the data unit #4 which is the data unit subsequent to “3”taken as the basis (as MSB), “0010” is placed in the Received MAP field.“0010” indicates the following: the reception of the data unit #4 failed(0); that of the data unit #5 also failed (0); that of the data unit #6succeeded (1); and that of the data unit #7 is failed (0).

When the data transmitting end receives this, it recognizes that thedata units of up to #3 were received and the data unit #6 was alsoreceived. In the next data transmission, the data transmitting enddeletes them from the transmit data unit candidate list, and furthertransmits the data units of sequence numbers “4,” “5,” “7,” and “8”based on the CTS information.

Thus, the RTS Sequence field and the RTS Received MAP field are used,and further multiplexing of the CTS SMH and the ACK SMH is usedtogether. Thereby, data can be transmitted and received without failure.

If the data transmitting end does not support selective acknowledgement,the following procedure can be taken: the transmitting end always placeszero in all the bits of the RTS Received MAP field, and, when itreceives an ACK and updates the transmit data unit candidate list, itdisregards the Received MAP. If the data receiving end does not supportselective acknowledgement, it can disregard the RTS Received MAP. Thus,even if communication is carried out between a communication stationwhich supports selective acknowledgement and a communication stationwhich does not, the communication can be maintained without failure.

H. Utilization of CTS Stored Sequence and Uses of Fields with RTS/CTSProcedure Used Together

FIG. 13 illustrates an example of data transmitting and receivingprocedures wherein delay ACK and the RTS/CTS procedure are applied. Thissection gives description with attention focused on the handling of eachsequence number and the use of the Stored Sequence field in the CTS SMH.

The example is on the assumption that the RTS Received MAP and ReceivedMAP fields are both constituted of four bits for the purpose ofsimplifying the explanation; however, the subject matter of the presentinvention is not limited to this constitution. Further, it is assumedthat the number of the Length fields in the RTS SMH is four, but thesubject matter of the present invention is not limited to this. Somevalue is indicated in the Duration field; however, no notice will betaken of the correctness of the value by Duration here.

With respect to RTS SMH, this example is also on the assumption that thefirst or fourth example of constitution is adopted.

Prior to data transmission, the data transmitting end transmits RTS(1).It indicates that the data units of sequence numbers “3,” “4,” “5,” and“6” are entered in the transmit data unit candidate list at this time.(That is, “3” is placed in the RTS Sequence field, and “0000” is placedin the RTS Received MAP field. Further, the data transmitting endnotifies that it desires that this data transmission should be completedin 255-unit time on the grounds of the data transmitting end. (That is,“255” is placed in the Max Duration field.) Further, the datatransmitting end notifies that the lengths of the data units it is aboutto transmit are 200, 20, 100, and 80 unit bytes, respectively. (That is,“200” is placed in Length0; “20” is placed in Length1; “100” is placedin Length2; and “80” is placed in Length3.) The time that lapses beforethe transmission of a CTS transmitted in response to this RTS iscompleted is set to 40-unit time, and “40” is written in the Durationfield.

After receiving this, the data receiving end returns CTS(1). At thistime, it determines rate class 1 as the transmission rate for datareception in accordance with the above-mentioned procedure, and notifiesof this. (That is, “1” is placed in RATE). Further, the data receivingend determines that it receives the data units #3, #4, #5, and #6 inaccordance with the above-mentioned procedure, and notifies that thetime period required for completing the reception of them is 240-unittime. (That is, “240” is placed in Duration.) Further, the datareceiving end holds the lengths of the data units of up to #6 which itdetermined to receive in this transaction. Then, it notifies that it hasheld information on the lengths of the data units of up to #6. (That is,“6” is placed in Stored Sequence.)

After receiving this, the data transmitting end transmits Data(1). Atthis time, it recognizes that it can transmit the data units #3, #4, #5,and #6 at the specified transmission rate class 1, in accordance withthe above-mentioned procedure. Then, it transmits these information attransmission rate class 1. Since delay ACK is specified, “0” is placedin the Duration field. Further, the data transmitting end holds sequencenumber 6 reported by the Stored Sequence in the CTS so that it can referto the number when it transmits an RTS next time.

It is assumed that the data receiving end can extract only the dataunits of sequence numbers “3” and “6.” However, the data transmittingend cannot confirm whether data reception succeeded or not, at thispoint of time.

To further transmit data units, the data transmitting end transmitsRTS(2). At this time, the data units of sequence numbers “3,” “4,” “5,”“6,” . . . are entered in the transmit data unit candidate list. WhenCTS(1) was received before, it was reported by Stored Sequence that “thelengths of the data units of up to sequence number 6 are held.”Therefore, information is described which is related to the data unitsof higher sequence numbers than 6 in the transmit data unit candidatelist. More specifically, the data transmitting end notifies that thedata units of sequence numbers “7” and “8” are entered and also notifiesof their lengths. (That is, “7” is placed in the RTS Sequence field;“0000” is placed in RTS Received MAP; “160” is placed in Length0; “120”is placed in Length1; “00” is placed in Length2; and “0” is placed inLength3.) That Length2 and Length3 are 0 notifies that transmit datacandidates are only two data units with lengths of 160 and 120. Further,the data transmitting end notifies that it desires that this datatransmission should be completed in 255-unit time on the grounds of thedata transmitting end. (That is, “255” is placed in the Max Durationfield.) Further, the time that lapses before the transmission of a CTStransmitted in response to this RTS is completed is set to 40-unit time,and “40” is written in the Duration field.

After receiving this, the data receiving end returns CTS(2). At thistime, it notifies that it determined rate class 2 as the transmissionrate for data reception, in accordance with the above-mentionedprocedure. (That is, “2” is placed in RATE in CTS SMH.) Further, thedata receiving end determines that it receives the data units ofsequence numbers “4,” “5,” and “7” in accordance with theabove-mentioned procedure, and notifies that the time period requiredfor completing the reception of them is 200-unit time. (That is, “200”is placed in Duration.) Further, the data receiving end holds thelengths of the data units of up to #7 which it determined to receive inthis transaction. Then, it notifies that it has held information on thelengths of the data units of up to #7. (That is, “7” is placed in StoredSequence in CTS SMH.) Further, the data receiving end determines toreturn ACK information as well, in accordance with the above-mentionedprocedure, and notifies that it could completely receive the data unitsof up to #3 and could receive the data unit #6. (That is, “3” is placedin ACK Sequence in ACK SMH, and “0010” is placed in Received MAP.)

After receiving this, the data transmitting end transmits Data(2). SinceACK information is included, at this time, it updates the transmit dataunit candidate list in accordance with the above-mentioned procedure.Thereafter, the data transmitting end recognizes that it can transmitthe data units #4, #5, and #7 at the specified transmission rate class2, and transmits them. Further, since delay ACK is specified, “0” isplaced in the Duration field. Further, the data transmitting end holdssequence number “7” reported by the Stored Sequence in the CTS so thatit can refer to the number when it transmits an RTS next time.

To further transmit data units, the data transmitting end transmitsRTS(3). At this time, it describes information related to the data unitsof higher sequence numbers than 7, which is the Stored Sequence heldwhen CTS(2) was received, in the transmit data unit candidate list. Morespecifically, the data transmitting end notifies that the data unit ofsequence number “8” is entered and also notifies of its length. (Thatis, “8” is placed in RTS Sequence; “0000” is placed in RTS Received MAP;“120” is placed in Length0; “0” is placed in Length1; “0” is placed inLength2; and “0” is placed in Length3.) Further, the data transmittingend notifies that it desires that this data transmission should becompleted in 255-unit time on the grounds of the data transmitting end.(That is, “255” is placed in Max Duration.) Further, the time thatlapses before the transmission of a CTS transmitted in response to thisRTS is set to 40-unit time, and “40” is written in Duration.

Thus, the transmitter and the receiver set the fields in the respectivepacket headers, and refer to each other's headers and thereby determinethe details of processing as the outcome of receive packets. As aresult, even if delay ACK is used together, Length information isprevented from being uselessly specified in RTS, and thus the efficiencyis enhanced.

I. Transmitting and Receiving Procedures for Traffic through TCP

FIG. 14 illustrates an example of the sequence in which traffic istransmitted and received through TCP/IP in accordance with the MACprocedure according to the present invention. Even if communication isof one way in terms of application, ACK is returned by the TCP layertypically every two segments, and it is brought into the form ofasymmetrical two-way communication in the MAC layer. The figureillustrates an example in which delay ACK is applied; multiplexing ofsuch packets as RTS, CTS, DATA, and ACK is permitted; and these are usedtogether.

In conventional transmitting and receiving procedures through TCP,packets equivalent to 24 times in total are transmitted and received inthe MAC layer to transmit five segments Data0 to Data4, as illustratedin FIG. 20. Thus, the processing is complicated and redundant.

In the transmitting and receiving procedures according to the presentinvention, packets equivalent to 15 times in total are transmitted andreceived in the MAC layer to transmit seven segments Data0 to Data6, asillustrated in FIG. 14.

As is obvious from the foregoing as well, it will be understood that, inthe transmitting and receiving procedures according to the presentinvention, the process in the MAC layer is simplified, and its overheadscan be significantly reduced.

Up to this point, the present invention has been described in detailsreferring to the specific embodiments. However, it is obvious that thoseskilled in the art can make modifications to or substitutions for theembodiments without departing from the scope of the present invention.

In this specification, description has been given to the embodimentsmade by applying the present invention to a system wherein the RTS/CTSprocedure and the access method based on CSMA are used together in awireless network in accordance with the IEEE802.11 standard. However,the subject matter of the present invention is not limited to this. Forexample, the present invention can be similarly applied to the followingsystems: a system wherein the RTS/CTS procedure is used together with arandom access method other than CSMA; and a random access system inaccordance with other requirements than in IEEE802.11.

In short, this specification discloses the present invention in the formof exemplification, and the contents of this specification should not beinterpreted in a restrictive manner. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A wireless communication system in which a data transmittingcommunication station acquires transmission right on a medium and thencarries out data communication with a data receiving communicationstation, wherein, after data transmission from the data transmittingcommunication station is stopped, it is detected whether the subsequentdata is present in the data transmitting communication station or not,and, if the subsequent data is present, it is made easier for the datatransmitting communication station to acquire transmission right fortransmitting the subsequent data.
 2. The wireless communication systemaccording to claim 1, in which medium access control wherein eachcommunication station confirms that said medium is clear only for apredetermined time, waits for an arbitrary backoff time, and thenacquires transmission right is carried out by each communicationstation, wherein, when the data receiving communication station detectsthe presence of the subsequent data in the data transmittingcommunication station, the data receiving communication station tries toacquire transmission right on said medium, and transmits a signal ontosaid medium for eliminating other communication stations' transmissionright and thereby makes it easier for the data transmittingcommunication station to acquire transmission right for transmitting thesubsequent data.
 3. The wireless communication system according to claim1, in which medium access control is carried out by each communicationstation concomitantly using the RTS/CTS method wherein a communicationstation as the origin of data transmission transmits a request to sendpacket RTS and starts data transmission in response to the reception ofa response packet CTS from a communication station as the destination ofdata transmission, wherein, when the data receiving communicationstation detects the presence of the subsequent data in the datatransmitting communication station, the data receiving communicationstation transmits a dummy RTS or an RTS onto said medium for eliminatingother communication stations' transmission right, and thereby makes iteasier for the data transmitting communication station to transmit anRTS for transmitting the subsequent data.
 4. The wireless communicationsystem according to claim 1, wherein the presence or absence of thesubsequent data in the data transmitting communication station isdetected based on the history of communication between the datatransmitting communication station and the data receiving communicationstation in the past.
 5. The wireless communication system according toclaim 1, wherein the presence or absence of the subsequent data in thedata transmitting communication station is detected based on that an RTSfrom the data transmitting communication station reaches the datareceiving communication station and the data receiving communicationstation returns a CTS but data transmission from the data transmittingcommunication station is not started, or that transmit data dose notreach the data receiving communication station.
 6. The wirelesscommunication system according to claim 1, wherein the presence orabsence of the subsequent data in the data transmitting communicationstation is detected based on specific information elements indicatingthe presence or absence of the subsequent data, contained in the datatransmitted from the data transmitting communication station.
 7. Awireless communication device which operates in a communicationenvironment in which each communication station acquires transmissionright on the medium before carrying out data communication, comprising:a communicating means which transmits and receives data on said medium;a data processing means which processes transmit/receive data; and amedium access controlling means which controls access operation on saidmedium, wherein, after data transmission from the data transmittingcommunication station is stopped, said data processing means detects thepresence or absence of the subsequent data in the data transmittingcommunication station, and wherein, if the subsequent data is present,said medium access controlling means makes it easier for the datatransmitting communication station to acquire transmission right fortransmitting the subsequent data.
 8. The wireless communication deviceaccording to claim 7, wherein said medium access controlling meanscarries out medium access control wherein it confirms that said mediumonly for a predetermined time, waits for an arbitrary backoff time, andthen acquires transmission right, and, if it detects the presence of thesubsequent data in the data transmitting communication station, tries toacquire transmission right on said medium, transmits a signal to saidmedium for eliminating other communication stations' transmission right,and thereby makes it easier for the data transmitting communicationstation to acquire transmission right for transmitting the subsequentdata.
 9. The wireless communication device according to claim 7, whereinsaid medium access controlling means carries out medium access controlconcomitantly using the RTS/CTS method wherein a communication stationas the origin of data transmission transmits a request to send packetRTS and starts data transmission in response to the reception of aresponse packet CTS from a communication station as the destination ofdata transmission, and, if it detects the presence of the subsequentdata in the data transmitting communication station, transmits a dummyRTS or an RTS onto said medium for eliminating other communicationstations' transmission right, and thereby makes it easier for the datatransmitting communication station to acquire transmission right fortransmitting the subsequent data.
 10. The wireless communication deviceaccording to claim 7, wherein said data processing means detects thepresence or absence of the subsequent data in the data transmittingcommunication station based on the history of communication with thedata transmitting communication station in the past.
 11. The wirelesscommunication device according to claim 7, wherein said data processingmeans detects the presence or absence of the subsequent data in the datatransmitting communication station based on that it returns a CTS inresponse to an RTS from the data transmitting communication station andthereafter the transmission of transmit data from the data transmittingcommunication station is stopped.
 12. The wireless communication deviceaccording to claim 7, wherein said data processing means detects thepresence or absence of the subsequent data in the data transmittingcommunication station based on specific information elements indicatingthe presence or absence of the subsequent data, contained in the datatransmitted from the data transmitting communication station.
 13. Awireless communication method in a communication environment in whicheach communication station acquires transmission right on the mediumbefore carrying out data communication, comprising: a step in which thepresence or absence of transmit data from other communication stationsis detected; and a medium access control step in which, in response todetection of the presence of transmit data from another communicationstation, a signal is transmitted onto said medium for eliminating othercommunication stations' transmission right so as to make it easier forthe communication station to acquire transmission right for transmittingdata.
 14. A computer program described in a computer-readable format forcarrying out on a computer system the processing for controllingcommunicating operation in a communication environment in which eachcommunication station acquires transmission right on the medium beforecarrying out data communication, the computer program comprising: a stepin which the presence or absence of transmit data from othercommunication stations is detected; and a medium access control step inwhich, in response to detection of the presence of transmit data fromanother communication station, a signal is transmitted onto said mediumfor eliminating other communication stations' transmission right so asto make it easier for the communication station to acquire transmissionright for transmitting data.